NA-73X reworked

A Radical Design

Four key factors made up the largest part of the success of the NA-73 model, which we will discuss here. For a more detailed look into the NA-73 design and details, please refer to the Mustang I article.

The largest contributing factors to the design of the NA-73 were:

  • Engine and cowling design
  • Use of the NACA high speed airfoil wing design
  • Use of the "Meredith" effect cooling system
  • Designed with regards to easy mass production

Engine and cowling design

The latest inline engine available at that time was the Allison V-1710 model, known as the F3R. This was the engine that had also been scheduled for the production of the Curtiss P-40D model. The Allison had a geared supercharger but power (and consequently, performance) was lost rapidly above 15,000ft. At that time the USAAC had no need for high-altitude combat fighters, so all American built fighters of that era were all equally llimited.

Radial engines have a large cross section as the cylinders "radiate" outward from a central crankcase.  The biggest disadvantage of radial engines is the amount of drag they produce, as all cylinders need to be exposed to the airflow to cool them.

Liquid cooled inline engines were necessary to produce faster airplanes as they provide a significantly lower cross section and, as a result, a far less amount of drag.

The use of liquid cooled engines prompted critisim from the military due to the vulnerability of airplane. Even the slightest damage to the coolant systems could result in the loss of the aircraft and its pilot. The logic answer to this critisism was the performance of the airplane and its ability to out-maneuver the enemy in combat.

Liquid cooled engines had also already proven themselves with British and German designs.

The sleek design of the cowling was courtesy of former race pilot Art Chester (the same man the NA-73 was designed around). Since NAA wanted to build the fastest aircraft possible, they began seeking to minimize drag wherever possible, starting with the front. The nose was given a tight fitting around the engine which resulted in a beautifully streamlined engine cowling. Only the carburator air intake broke the sleek lines at the front.

NACA high speed airfoil

Another design feat that would ensure the NA-73X was an extremely fast fighter was the use of a brand new and, at that time revolutionary wing design: the laminar flow wing.

A very important factor of new fighter aircraft design was the wing, since the wing design is the toughest of all components and usually determines the schedule for the rest of the design job. The wing did not only have to perform well, produce as low drag as possible and handle high-stress loads from combat maneuvers and payloads, but also had to house the guns, ammuniation, fuel tanks and the landing gear assembly.

In the years leading up to the war up to that point, the go-to wing design was the NACA 23000 series, which is still being used today.  

On the wing

A bit of wing history for those interested: when the Wright Brothers took to the skies in their Wright Flyer, the wing was nothing more than a straight wing, with a very small camber. 

When aviation design picked up in the first World War, a German aerodynamic researcher named Ludwig Prandtl began experimenting with various designs in a wind tunnel at Göttingen. He discovered that thicker wing sections, wiht larger leading-edge radii allowed higher angles of attack, and thus produced more lift before stalling. 

During the late 1920s, early 1930s the US National Advisory Committee for Aeronautics (NACA) developed and tested several "families" of airfoils. These airfoils were give numerical designations which described the profile of the wing.

The NACA four digit wing sections define the profile by:

  • First digit describes maximum camber as per percentage of the chord.
  • Second digit describes the distance of camber from the airfoil leading edge in tens of percent of the chord.
  • Last two digits describe maximum thickness of the airfoil as percent of the chord.

For instance, the NACA 0018 airfoil is a symmetrical one:

  • The first 0 indicates that there is no camber (0%)
  • The second 0 indicates there is no distance of camber
  • The last two digits indicate that the airfoil has a 18% thickness to the chord length ratio; it is 18% as thick as it is long.

The most common used airfoil in the late 1930s, the 23012 is a five-digit airfoil.

In this code, the first number still indicated the maximum camber in percent of the chord and the last two numbers still indicated the maximum thickness in percent of the chord; however, the middle two numbers indicated the position of maximum camber in percent of the chord rather than the previous single number in the four-digit series indicating maximum camber in tenths of the chord.

Furthermore, the five-digit (and subsequent six-digit) series also indicated modifications like changes of the leading-edge radius or the position of maximum thickness by adding a suffix consisting of a dash and two more digits, as with N.A.C.A. 23012-64, one of the most outstanding sections in the popular 230-series, the family announced in 1935. 

The last two shapes (N.A.C.A. 661 -212 and N.A.C.A. 74 7A315) are low-drag sections designed to have laminar flow over 60 to 70 percent of chord on both the upper and the lower surface. Note that the laminar flow sections are thickest near the center of their chords. 

With the earlier wing designs, once the airflow (commonly referred to as boundary layer) passed the thickest point of the wing, it had a tendency to become turbulent. This turbulence became an extra drag for the aircraft moving forward.

The laminar wing design has a far different airfoil section from that of a conventional wing in that its thickest point is further back. This in turn creates different airflow characteristics over the wing in which the flow of air remains ‘attached’ to the wing for a larger part of the wing's chord than with a normal wing.

This less turbulent portion on the wing results in a small drag reduction, as well as the absence of pressure peaks allows higher speeds to be reached before the compressibility burble is encountered.

The early work on laminar-flow airfoils is was first described by aerodynamicist Eastman N. Jacobs in NACA confidential report  document "Preliminary Report on Laminar-Flow Airfoils and New Methods Adopted for Airfoil and Boundary-Layer Investigations", originally published in June 1939.

© Steve Karp

According to Lee Atwood, the original concept did not include the laminar flow wing.  

On June 17th, 1940, G. W. Lewis, Director of Aeronautical Research, NACA wrote a letter to Lt. Col. Carl F. Greene informing him that letters have been written to the chief engineers of several aircraft companies, informing them on the laminar-flow wing, and inviting them over to Langley Field to consult with their technical staff. (20)

North American had by then received preliminary NACA reports on the low-drag airfoils, as had all the other companies building aircraft for the army and navy. NAA's engineers, especially Ed Horkey, were favorably impressed. He relied heavily on recent NACA Technical Reports of the new low-drag/high-speed airfoil wind tunnel tests at the GALCIT, which resulted in astonishing low-drag properties.

Within the company, and particularly with Dutch, there was a great deal of concern about selecting a brand new, untried type of airfoil. More so because of the tight prototype delivery and production deadline.

However, so much enthusiasm for the laminar-flow wings sprouted among NAA engineers, that the company ultimately stuck its neck out and selected one of the NACA’s Series 4 airfoils. In hindsight, NAA's decision to try the laminar flow concept on its first chance of fighter design seems a tremendous risk. At that time, only the data from Jacobs’ advance confidential report was available. There was a substantial chance that the  wing could have had poor stall or stability characteristics, or any other number of unforeseen problems. (21)

As Schmued recalled:  "We had planned to use an NACA-23 series airfoil and then heard NACA had developed a laminar-flow airfoil. That airfoil was specifically adapted by Ed Horkey, our first and only aerodynamicist and his assistants, who developed the ordinates of the laminar-flow airfoil.” 


Kindelberger was concerned that this new wing might not work, but Edgar assured him that he would design and build another wing in just one month.

Regarding the wing, Ed Horkey later wrote, “Our concern was that we didn’t get all of the great laminar flow projected, since 20 percent would be too thick an airfoil, even in those days. It was decided to lay out new airfoils, which were around 16 percent thick at the root and 11 percent thick at the tip...


In other words, what we did is pick the pressure distribution we wanted. Then we drew an airfoil shape, and... we could check our pressure distributions. If it didn’t match, we could make a change to the airfoil contour, then go back and recalculate the pressure distribution.” 

Schmued wrote, “We discovered that when the wing was very thin, the aileron control system had to be extremely well designed to fit into the small space that was available. We used some rather unorthodox systems for the aileron control by using a wobble plate. That is a term for a special form of mechanism which we used that was very successful." (22)

Both NAA and NACA were working on developing a new, thinner, laminar flow wing that would work for the NA-73X. 

A $20,000 one-quarter-scale mahogany model of the NA-73, lacquered and rubbed down to a high polish and practically identical with the future prototype, was constructed and brought to Cal Tech by Ed Horkey.  After initial testing in the wind tunnel, a great sigh of relief was felt with NAA engineers as the wing had been tested up to transonic speed and it indicated a drag factor of less than 50% of any wing design previously tested.

Their joy was short-lived however as at slow speeds the stalling characteristics proved to be very bad.

Calculations and modifications were made the following week and a new model was built and tested at Cal Tech. Once again, the the wing did not work as expected. After closer examination, the engineers suspected that the wind tunnel itself might me the problem as it was too small and the tunnel walls were affecting the air flow at the wing tips.

The model was loaded into an airliner and flown to a larger wind tunnel at the University of Washington in Seattle, accompanied by Horkey. He later jokinly made the remark that the wing's first flight was inside a plane.

In the larger tunnel, the earlier hypothesis was proven correct as the wing passed the test with flying colors. (23)

The wind tunnel testing was done between June 28th and July 26th, 1940.

A small disadvantage on the laminar flow wings is that it provides less lift at lower speeds. In order to compensate, the NA-73X was given large, powerful slotted flaps to keep landing speeds from being impractically high.

To further ensure a smooth airflow, fillets were formed to smooth out the junctions of the wing and tail surfaces where they intersected the fuselage. Everything was secured with countersunk screws recessed into washers which provided a flush surface. All of the riveted skin was countersunk riveted providing a smooth finish.

The removable panels for gun and ammunition bay acces were secured with Dzus fasteners (a plain slot head locking pin which fit into a recessed collar and attached to a spring under tension when rotated 90 degrees. The springs were attached to the support structure in such a manner that the slot in the head of the locking pin was parallel to the structural member which was secured. This method provided a means of visually checking the' security of all panels before flight). (24)

Also notworthy: this wing shape would remain the same all through the Mustang's lifetime: a 37.03-foot span and 233.42- square-foot area, with chord tapering from 104 inches at the root to 50 inches at the tips. (25)

Meredith effect cooling system

The use of an inline liquid cooled engine requires a large radiator, the placement of which can greatly affect the aircraft's performance. On the Spitfire for example, the coolant radiators are located beneath the wings. This design has a few drawbacks: firstly, when on the ground, the intake of the radiator was partially blocked by the undercarriage which caused temperature problems while running the engine on the ground. Secondly, if a stray bullet went through the radiator, the engine would overheat which lead to a forced landing. Thirdly, both radiator intakes produce extra drag for the fighter.

NAA's design team took its inspiration from the 1939 NACA Study No. 896 "The Drag of Airplane Radiators with special reference to air heating", which was, as mentioned above, a translation of B. Gothert's 1938 German report.

© Steve Karp

The Meredith effect is a phenomenon whereby the aerodynamic drag produced by a cooling radiator may be offset by careful design of the cooling duct such that useful thrust is produced. The effect was discovered in the 1930s and became more important as the speeds of piston-engined aircraft increased over the next decade.

The Meredith effect occurs where air flowing through a duct is heated by a heat-exchanger or radiator containing a hot working fluid such as ethylene glycol. Typically the fluid is a coolant carrying waste heat from an internal combustion engine.

For the effect to occur, the duct must be travelling at a significant speed with respect to the air. Air flowing into the duct meets drag resistance from the radiator surface and is compressed due to the ram air effect.

As the air flows through the radiator it is heated, adding heat energy to the air and further increasing its volume. The hot, pressurised air then exits through the exhaust duct which is shaped to be convergent, i.e. to narrow towards the rear. This accelerates the air backwards and the reaction of this acceleration against the installation provides a small forward thrust. The thrust obtainable depends upon the pressure difference between the inside and outside of the duct. The air expands and cools as it passes along the duct, before emerging to join the external air flow.

F. W. Meredith was a British engineer working at the Royal Aircraft Establishment (RAE), Farnborough.

The location of the coolant installation beneath and behind the cockpit served multiple purposes:

  • To establish the drag factor as far aft as possible and behind the wing foil
  • to provide a better center of balance for the airframe
  • The propeller slipstream provided an airflow during ground operation and taxi periods to prevent overheating

The actual development to achieve the promised “zero net drag” took a considerable amount of time. Several tests and adjustments were made to the position and shape of the front scoop, radiator position, boundary-layer seperation and turbulence aft of the radiator, with exit temperatures above 170ºF.  As Horkey stated: “The boundary-layer bleed wasn't all that simple. If you drop the radiator inlet down far enough to get rid of all of the turbulent boundary layer, the drag would be too high; so it ended where it was a compromise of the bleed depth to get an acceptable cooling performance with minimum drag."

Some of those refinements included:

  1. The intake scoop had to be designed in such a way to smooth out the air entering the forward upper and lower plenum before going through the radiator and oil cooler.
  2. The speed of the air entering the radiator had to be reduced as much as possible (this resulted in the boundary-layer seperation or the space between the top of the scoop and the airframe), in order for the radiator to optimize its cooling.
  3. The air had to be efficiently compressed once exciting the radiator/oil cooler to build up pressure before exciting
  4. The rear scoop had to be designed in order to be able to open and close (to regulate the temperature of the radiator coolant)

The feature that was not included in the original design of the X73 was any initial attempt to create a “gutter effect” to prevent boundary-layer air from entering the front scoop.

It was estimated that around 90 percent of the cooling system drag would be eliminated by the efficient thrust of the exiting air.

Early tests revealed that the engine was not being cooled adequately. Further wind-tunnel tests showed that the disturbed boundary layers of air beneath the wing and fuselage prevented a “clean” flow through the radiator.

Schmued recalled that the British Air Ministry was extremely helpful in the development: "Among others, they sent us Dr. B. S. Shenstone, who arrived February 25th, 1941, to assist us in some of the airflow problems into the radiator. The radiator, as we had it, consisted primarily of a fairing, which started at the bottom of the fuselage and enclosed the radiator. Dr. Shenstone advised us to provide an upper lip on the radiator housing, which was about 1% inches below the fuselage contour. By doing this, we got a much better pressure distribution in the air scoop."

The entrance lip of the scoop was lowered one inch. The turbulent air was thus bypassed and perfect air circulation was attained. (26)

The lines and radiator served as a closed system for the engine cooling fluid. In those days, ethylene glycol had been adopted for use in liquid cooled engines due to the wide temperature range between the freezing and boiling points. The capacity of the fluid to dissipate heat required less radiator surface area than water and this factor allowed the adjustable scoop inlet to be fitted closely against the underside of the fuselage.

Please note that the images below are taken from the P-51-NA manual and do not directly apply to the NA-73X prototype! They do give a general image of the coolant system.

Designed for mass production

Other airplanes of the same period, such as the P-40, were not designed to enable easy mass production. As Dutch Kindelberger stated: “The time to start worrying about pro­duction is before you begin product engineering”.

As Atwood recalled: "Dutch Kindelberger put a lot of effort and talent into increasing the efficiency of airplane production. Even at high wartime rates of production, parts were made in batches, and it was most unusual to have a machine tool dedicated to making one part, or even to one operation. Many tools, especially for sheet metal parts, were "soft” tooling, using masonite, plywood, or low temperature casting materials rather than tool steel, and were much cheaper— if not as durable. However, they were adequate for the purpose, were made very much more quickly, and were adaptable to the inevitable changes that came along. Dutch made many contributions to the cutting, forming, and stretch-forming techniques, but his greatest improvement came from rationalization of assembly and installation processes." (27)

Up to that time it was common practice to finish the structural elements, wing, fuselage, etc. and after that start with the installation of electrical and hydraulic equipment, armament, instruments, and other items, in the nearly completed structure. In large airplanes, with plenty of access room, this worked reasonably well with few bottlenecks, but in smaller planes, such as fighters and trainers, the final assembly stage was crowded, hectic, and inefficient. Installing various systems whilst hanging upside down inside the cockpit was often difficult and time consuming.

Starting with Harvard production, Dutch required that fuselage and wing structures remain open in sort of half-shell condition until all wiring, tubing, and permanent equipment installations were made and that they be inspected and tested before joining into complete structures This naturally required that the engineering design provide for this construction process. Therefore plumbing and cable disconnects were employed to facilitate assembly. From that point on it became part of house practice in all production models.

So was the case even in the development design of the new fighter: the assembly lines were well organized and planned as each aircraft section was detail designed. NAA's production line was similarly arranged as an automobile production line with the aircraft sub-assemblies moving through various stations for installation of respective systems and components. 


They envisioned a process in which subsystems on both the right hand side and the left hand side of the fuselage were assembled before both halves were joined. The whole assembly was cut into five seperate subassemblies:

  • the engine compartment forward of the firewall
  • forward fuselage section
  • wings
  • aft fuselage section (first right and left hand sides seperately, then joined to finish)
  • empennage with tail cone

This out of the box thinking not only facilitated the construction process, they also had field maintenance in mind as several easy access panels were built in.

Once the fuselage halves were joined together, the empennage, wings and powerplant were installed, along with “pluggable” electrical, hydraulic, and fuel lines subsystems when all major components were brought together. The fuselage itself was one of the smallest cross-sectional area ever to be placed be­hind an Allison engine and demanded some high-density packing of the equipment (radios, com­passes, hydraulic systems, ....) that had to go into it. Or as one NAA engineer put it: “It’s like stuffing the insides of a bomber into a fighter".


The wings were assembled on a seperate line and were attached to the entire forward section at four points. Both the horizontal and vertical tail surfaces were fitted to the aft section.

Art Chester designed a new cantilever engine mount using a riveted aluminum-box beam structure, instead of the usual welded steel-tube framework. This new mount provided considerable rigidity to the design. The complete mount weighed less than 200 pounds and provided an attachment point for cowling, ducting and the two fifty caliber machine guns which would be synchronized to fire through the propeller arc. The entire engine assembly with accessories could be built-up for installation onto the fuselage frame at the appropriate assembly station. (28)

The stabilizer, fin and wing tips were also squared off. Main reason was also with an eye on mass production. As a bonus, the calculations used in design indicated that the of the laminar foil adapted to the square tip planform with a lesser degree of turbulence being present than with a curved foil.

Further major design features were:


  • Metal skin: NA-73X had an all metal skin except for the rudder and elevators which were fabric-covered. The stressed aluminum skin also had all flush riveting or screws for speed.

  • 2 large self-sealing fuel tanks, one 90 US gallon tank in each wing, that could hold about double the fuel a Spitfire carried. The left hand tank was divided into two sections: the smaller of which held 32 gallons and was selective as a reserve tank.

  • A wide track landing gear that would retract inwards into wheel wells in the wing forward leading edge, the leading edges kinked forwards at the fuselage join to provide sufficient room.


    This resulted in far better ground handling then a Me-109 or Spitfire had. When the gear was down, the inner gear doors would also retract to keep drag to a minimum.

  • A fully retractable and steerable tail wheel.

  • 8mm thick armor behind the pilot's seat

  • The front of the “Turtleback” canopy was armored glass and had a hinged panel on the left and hinged panel on top. The panel could be jettisoned in flight in case of emergency.

  • A 10-foot-6-inch (3,2 meter) diameter Curtiss Electric 3-blade propeller

  • The wings were designed to allow a single bomb or other store to be carried on a single pylon underneath.

  • The Brittish specs included heavy armament of 8 guns. NAA planned to use four .50 cal and four .30 cal guns. Two .50 cal guns would be installed in the nose with syncro firing through the propeller, the other two in the wings along side the .30 cal guns. The wing mounted guns were staggered for a better fit.


How it all began

North American Aviation

NAA's roots can be traced back to December 6th, 1928, when financier Clement Melville Keys ran a holding company for several different aviation companies, including Douglas and Curtiss. However, the Air Mail Act of 1934 forced the breakup of such holding companies. A year earlier the General Motors Corporation purchased a stake in North American Aviation and merged it with the General Aviation Manufacturing Corporation. The company retained the name North American Aviation or NAA in short.


Thus NAA became a manufacturing company, of which James H. "Dutch" Kindelberger was put in charge. Kindelberger had been recruited from the Douglas Aircraft Company.

NAA built a brand new factory at Inglewood, near Los Angeles, and personnel were selected from manufacturers within the old group (mainly from the General Aviation Manufacturing Corporation, who built Fokker aircraft in Baltimore, Maryland). The company's operations then moved from Dundalk, Maryland to Los Angeles, California.


Mines Field (now LAX International Airport) was picked as NAA’s new location since it offered an established airfield that was not only close to supply sources, but was also cheap (NAA’s original 20-acre site was leased to the company for some $600 per year!).

Another big advantage was the weather, which allowed for almost year-round flying. The new NAA complex was opened in 1936 and covered 159,000 sq ft, while 150 employees were on the payroll.

James H. "Dutch" Kindelberger © Smithsonian Air and Space Museum

James H. "Dutch" Kindelberger (8 May 1895 – 27 July 1962) dropped out of school in the 10th grade and started working in the steel industry with his father. He took correspondence courses to further his education. In 1916, when he was 21 years old, he went to study at the Carnegie Institute of Technology to pursue engineering.


During World War I, he was a member of the US Army Air Service, where he was a pilot instructor based at Park Field in Memphis, Tennessee. After the war, Kindelberger looked for work in aviation and , in 1920, became chief draftsman and assistant chief engineer with the Glenn L. Martin Company in Cleveland, Ohio.


Kindelberger moved to the Douglas Aircraft Company in 1930 where he worked as an engineering executive. At was there that he met John Leland "Lee" Atwood when the two of them worked the DC-1 and DC-2 transports.


Both men left Douglas Aircraft in 1934, moving to North American Aviation. There, Atwood assumed the title of Chief Engineer and Kindelberger was named President and General Manager. When they both started at NAA, the company had orders for one passenger aircraft. That Kindelberger proved to be a good businessman showed in the fact that NAA managed to get a $1 million order for a military trainer, the North American BT-9.

He was famed for his emphasis on hard work, orderliness and punctuality. His down-to-earth manner made him a very easy character to work with and he would lead NAA until 1960.


His nickname, 'Dutch' referred to his descent from German (Deutsch) immigrants from Nothweiler, Pfalz.

The US Army Air Corps grew out of the Air Service in 1926. In an effort to strenghten the Army Aviation branch, the US War Department General Staff approved a five-year expansion program. The goal of this program was to increase the number of tactical squadrons from the existing 32 to 63. It was later adjusted to 52 squadrons and comprised expansion to 1800 aircraft, 1650 officers and 15000 enlisted men. The start of the program was delayed to July 1st, 1927, but five years later, in July of 1932, none of the goals were reached. (1)


The Air Corps did double from 7 to 15 groups by the end of 1932:

USAAC Groups after  5-year expansion program © Crazy Horse Aviation Photography

During the late 1920s and early 1930s, it was still a small branch of the ground forces. To give an idea: in 1934, the Army Air Corps consisted out of 15,861 people (approximately 11.5% of the entire US Army strenght). At its peak in 1944, it would grow to 2,372,292 personnel or almost a third of the US Army strength. (2)

Up to 1935, most Pursuit Groups were equipped with Curtiss P-1 Hawk and Boeing P-12 and P-26 Peashooters.

In contrast to the development of fighter (pursuit) aircraft was the development of bombers: on July 22nd, the Boeing Model 299 (B-17) made its first flight. The following month, the prototype made a 2,100 mile flight from Boeing's plant in Seattle to Wright Field at an average cruise speed of 232mph. In doing so, it completely outclassed its competition and a bomber legends was born... it would take the pursuit aicraft several more years to come to full development status.

Several big things changed for the aerial branch of the United States Army in 1935: the creation of the General Headquarters Air Force brought forth the first official recognition that the development of effective airpower necessitated authority for independent action. The GHQ Air Force came to be because the War Department began to understand that aviation had introduced a new and increasinly important element into warfare. In contrast, the Army continued to insist that the Air Service had no other mission than the support of ground forces.

The 1926 Air Corps Act changed the name of the air arm, which made it sound more important, but its role and status remained unchanged. Billy Mitchell was a huge supporter of an independent Air Force, but his continuous drive towards such independence only led to him being court-martialed in 1925 for insubordination.

Finally, as tentions and command difficulties began to rise between the Army and its air department, and because bigger and better aircraft began to develop, proposals for an increase in budget for the Air Corps and for its independence were continuously being made. It was during the 1934 Baker Board that the suggestion came to permanently set up a GHQ Air Force. Finally on March 1st, 1935 the GHQ Air Force was set up, with its headquarters at Langley Field, for centralized control of aviation combat units within the continental United States, separate from but coordinate with the Air Corps. The Chief of the Air Corps, Maj. Gen. Benjamin D. Foulois tried to obtain command of the GHQ AF, but Army leaders were reluctant to give him any more power than he already had, so Frank M. Andrews was chosen as its commander.

Secondly, the procedure to acquire new hardware changed.

Following the Air Corps' brief stint in taking over the Air Mail in the US, it saw that a lot of civilian aircraft were far superior than planes developed specifically for the Air Corps. As a result, they sought to modernize its bomber and tactical combat force.

A system called "Request for Proposal", or RFP in short, was introduced. It works as a sort of solicitation in which certain requirements are stated to prospective contractors and to solicit for proposals. Once an RFP is released by a government agency, bidders can submit a proposal and attempt to win the work contract. Details of these specifications then appeared in the form of "Circular Proposals" '(CP), the designations of which were built as CP XX-YY, where XX corresponds to the fiscal year and YY is the order in which each year's projects were issued. The new system had one main characteristic: insistence on competition.

Once a contracter won the bid, the aircraft would be tested at the USAAC Materiel Command Division (USAAC-MD), which was under the direction of Col. Oliver Echols at that time. It was due to his efforts that the Air Corps had as much as it had when the situation in Europe became increasingly more unstable.

The experimental and production types were tested at Wright Field, Ohio. The test program was headed by:

  • 1st Lieutenant Benjamin S. Kelsey for pursuit types
  • 1st Lieutenant Leonard Harmon for bombers
  • 1st Lieutenant Patrick Timberlake for attack aircraft

One such RFP, CP 35-414, was made on January 14th, 1935, by the USAAC-MD (United States Army Air Corps, Materiel Division). The Circular Proposal contained the technical specifications for:

  • X-602 for a biplace, single-engined pursuit with bomb-carrying capability
  • X-603 for a single-seater, single-engined pursuit.

With the emergency of the Allison V-1710 in-line-engine, NAA decided to place a bid for the dual-seat X-602 project.

Vega 35 prototype by Lockheed, development of NA-35 © SDASM

© SDASM - The NA-35 design was never manufactured by NAA. Vega Aircraft Corporation bought the rights in October of 1940 to develop into the Vega 35 (Vega prototype was registered NX21760, whereas the NA-35 was NX14299)

After providing sufficient internal funding, the company came up with specification NA-35, which resulted in the P-198 XP design to compete in the X-602 contract.

NAA's Chief Engineer, Lee Atwood, presented the detail specifications and side profile drawings on April 4th . Side profile drawings for the P-198 were released on May 5th , 1935.

The airframe became the first NAA design to incorporate an in-line engine with a single-speed/single-stage supercharger. Unfortunately for NAA, the USAAC canceled the Proposal request before the company could submit their specifications.

What the designers at North American Aviation were unaware about what that USAAC-MD Captain (later Major General) Oliver P. Echols, never considered NAA to be a developer of pursuit aircraft.

Echols preferred the Seversky P-35 over the Curtiss P-36, both aircraft being conventionally powered by radial engines. (3)

A lucky break for NAA came when the USAAC started a new competition for a two-seat basic trainer shortly after. With the possibility to deliver a huge amount of training aircraft to the USAAC, GMC management released extra funding to NAA in order to compete in the trainer proposal.

NAA opened project number NA-16 on January 25th , 1935, which specified the design of a two-seat basic trainer with fixed gear. This became the basis for NAA's highly successful Harvard trainer.

The NA-16 prototype took to the skies for the first time in April of 1935. Paul Balfour made the first flight. The NA-16 was ferried to Wright Field on April 22nd, and was assigned as BT-9. NAA beat the Seversky BT-8 and consequently received a contract for 42 aircraft.

Following the contract on September 19th, NAA opened General Order NA-19 to provide funds for the production version of the BT-9.(4)

The contract for the BT-9 provided NAA with sufficient funds to move their business to Mines Field (now Los Angeles International Airport), as well as the construction of a new production plant in Inglewood.

Meanwhile, over in Germany, the Bf-109 made its first flight in October of 1935. On March 5th, 1936, the Supermarine Spitfire made its first flight from Eastleigh Aerodrome, flown by Captain Joseph "Mutt" Summers (Vickers Chief Test Pilot), and in June, the Hawker Hurricane went into full production.

NAA decided to focus on training aircraft, as other manufacturers were already making name for themselves regarding pursuit aircraft at that time. The idea of designing and producing a pursuit aircraft however, never escaped their minds.

Under the new acquisition system, several small, marginal, and even entirely new companies, or established companies that had not competed seriously before in fighter R&D, suddenly became major contenders.
For example, the little-known Seversky company (which later became Republic), founded only a few years earlier, won a major fighter competition in 1936 by beating a Curtiss design. Their P-35 was acquired by the USAAC and became it's first single-seat monoplane fighter with retractable landing gear and an enclosed cockpit.

This did not mean however, that established companies in the aviation industry, such as Boeing and Curtiss were faded out from the fighter business. At the time, Boeing commited much of their resources to the development of a long-range bomber (B-17). Although the Seversky entry was declared the winner and awarded a contract for 77 aircraft, Curtiss also received an order from the USAAC for three prototypes, designated Y1P-36.
Main reason was that the USAAC was concerned about the political situation in Europe, and about Seversky's ability to produce its aircraft in a timely manner. As such, they wanted a backup fighter.

Curtiss however continued their development of their Y1P-36 when asked by the USAAC to adapt one P-36 to the new liquid cooled, turbo-supercharged Allison V-1710 engine. The new design was designated the XP-37 and went on to win the 1937 USAAC competition with an order for 210 P-36A fighters. (5)

Unfortunately for the USAAC, all of those aircraft were already obsolete by the time they came into service, compared to their UK and German counterparts.

In the ongoing search for more capable pursuit aircraft, Lt. Benjamin Kelsey and Captain Gordon P. Saville, fighter tactics instructor at the Air Corps Tactical School, issued a specification for a new fighter via Circular Proposals CP37-608 (twin-engined pursuit) and CP37-609 (single engined pursuit), in February of 1937.

These proposals were sent to nine aircraft manufacturers, but, much to Kindelberger's surprise, his company was not amongst those receiving the proposals.

Lockheed, a small company that had become a recognized developer of commercial transports but had never successfully developed a military design,

won the -608 competition on June 23rd with its Model 22 and was contracted to build a prototype, the XP-38. The single engined -609 project winner would be the Bell Model 12, which had a unique configuration with an Allison V-12 engine mounted in the middle of the fuselage, just behind the cockpit, and a propeller driven by a shaft passing beneath the pilot's feet under the cockpit floor. The Bell Model 12 would develop into the P-39.

Construction for both aircraft began in early 1938, with the XP-39 already making its maiden flight on April 6th, 1938. The XP-38 would not fly until January 27th, 1939.


Several interesting things happened in 1938.

On january 28th, President Roosevelt declared the countries' national defenses inadequate in the face of warlike preparations abroad which constituted “a threat to world peace and security.” He asked for appropriations, largely naval, to improve US defenses. The itemized list of requirements included a budget for antiaircraft materiel but not for aircraft.

On February 2nd, the people at Allison contacted NAA regarding possible USAAC interest in their V-1710 configuration, boosted by a General Electric turbo-supercharger, for another upcoming pursuit competition.  The BC-1A (NA-36) of 1938 was North American's first combat aircraft and was based on the GA-16.  It carried a .30-cal M-2 machine gun on the starboard nose, and a flexible M-2 in the rear. On February 11th, test pilot Vance Breese flew the BC-1 for the first time.

Upon receiving some backdoor information from Allison, Kindelberger contacted Dr. Millikan at the GALCIT (Graduate Aerospace Laboratories of the California Institute of Technology), requesting a study for an optimized "High Speed Pursuit" based on a 1,050hp in-line engine.

In the UK, in early 1938, deliveries of military aircraft were about 150 aircraft per month. When Germany annexed Austria on March 13th, the British Air Ministry adjusted their goals and sought after an increased procurement of aircraft, which resulted in "Scheme L". This scheme envisioned a provision of 12,000 aircraft over a two year period, including fighters, bombers, reconnaissance aircraft and trainers.

There were a couple of issues with that plan: the British aircraft companies had never before produced aircraft at that scale and British aircraft were complex and time consuming to build. Existing designs were never created with mass production in mind.


The British industry could probably have obtained such production numbers, given enough time. As tensions were still rising however, and as Britain's armed forces were rapidly expanding, the British Air Ministry decided to send a delegation over to Canada and the US in February to look for potential contractors there, under the name British Air Mission. The delegation was led by A. T. Harris and Sir Henry Self.


Canadian resources were found to be slim and also lacked both means and capacity at that time. Across the border, in the US, the British delegation was impressed by the many aircraft manufacturers and the quality of the airplanes, especially the metal airframe constructions.

There was one drawback however: the US Neutrality Act, which ensured that the US would not become engaged in any foreign military conflict.
Even though the US was adopting a strict policy of neutrality, there were no objections to prevent friendly foreign countries from buying military products on a cash-and-carry basis, providing it was not actually engaged in any hostilities at the time.


The British were not the only "customers" seeking help overseas. The French Government was also urgently looking to strengthen its air force.  A French Air Commission led by M. Jean Monnet placed several contracts in the US.


Being impressed by NAA's NA-16 trainer series, the British Air Mission quickly put down an order for 200 aircraft (designator N-16-1E - E for England, NA charge number NA-49), on February 7th. They would name their new acquisition the Harvard Mk. I. (6)

Lockheed also received an order for 200 Hudson bombers. It was NAA's first major contract and would keep the Inglewood plant busy for several months.

When the Air Corps learned in April that British agents were engaged with huge aircraft contracts, they feared that the US aircraft manufacturers would delay deliveries to the Air Corps, instead of expanding their plants.

In May, the National Advisory Committee for Aeronautics (NACA) published Technical Report Memo 896. This was essentially an english translation of B. Gothert's German report "Drag of Radiator with Special Reference to Heating". The memo included several design concepts for diffuser, intake plenum, coolant radiator and the exit nozzle.

That same month, Kindelberger made a trip to Europe. The main goal for the trip was to try and find customers for NAA's Harvard trainers. While in Europe, he took advantage of visiting several European aircraft manufacturers and in doing so, assessed the state of overseas aircraft productions.  He departed England with an extra order for two hundred more Harvard trainers and a better understanding of the dismal state of Britain’s aviation industry. Their production rates were poor and their Spitfire was a very labor-intensive build.

Kindelerger was also invited to Germany by German World War I ace Ernst Udet. The two met at a 1936 airshow where Udet performed his trademark feat of daring by plucking a scarf from the Mines Field runway with his wing tip. During his ten-day stay in Germany, Kindelberger was granted access to the Heinkel, Focke-Wulf, and Messerschmitt plants, as well as a secret research lab. In great contrast to the British, he learned that German airplane designs were adapted for simple tooling and high-rate production. Why Kindelberger was given access to all that information remains a mystery. It is possible that it may have been an attempt to bluff the United States into staying out of the war.

When Kindelberger returned home, it became clear to him that their NA-53 based P-500 pursuit fighter concept was not going to cut it with the then cutting edge German fighter, the Bf 109. It would also be inferior to both the British Hurricane and Spitfire fighters.

Several other key dates in 1938:

  • In June, Edward Horkey joined NAA as aerodynamicist, by recommendation of Millikan.
  • On October 14th, the Curtiss XP-40, a further development of the Model 75, but now with the in-line Allison V-1710-19, flew for the first time. Two weeks later Lt. Ben Kelsey flew it from Dayton to Buffalo at an average speed of 350mph, setting a new USAAC speed record.
  • In November, NAA received a contract for the NA-50 export fighter. The company opened a new Charge Number, NA-68.

In September, Britain's prime minister Neville Chamberlain and France's president Douard Daladier, flew to Munich to meet with Hitler. To avoid war, they gave the Sudetenland to Hitler. Upon returning to Britain, Chamberlain happily anounced that he had helped to negatiate "peace for our time". On Wednesday, September 21st, Henry "Hap" Arnold received word that General Westover, chief of the Air Corps, had crashed flying his A-17. On September 29th, Arnold was given a new job opportunity and was sworn in as Chief of Staff of the US Army Air Corps, with the rank of Major General.

By the fall of 1938, following the escalation of events in Europe and the Pacific, President Roosevelt began to realize the importance of military aviation as a weapon of war and as a symbol of power. He held a top secret meeting in the White House on November 14th, where he came forward with a program to expand the Air Corps to Hap Arnold, Brig. Gen. George C. Marshall, the Deputy of Staff and a few other officials. Everyone was suprised when he spoke in terms of 20,000 airplanes and an aircraft industry production capacity of 24,000 aircraft per year.

This growth plan was very ambitious and easier said than done, because it not only encompassed the actuel aircraft capacity, but also a parallelled expansion of personnel, airfields, maintenance depots and training centers. At the time of the plan, the Air Corps only consisted of 2,058 regular officers, 669 reserve officers and 23,779 enlisted men.

By the end of 1938, NAA did gain a lot of recognition in the US and abroad because of their innovative airplane design and production company. The company delivered high quality projects such as the XB-21, BC-9, O-47, NA-50 and their Harvard I trainer. NAA's success with their Harvard trainer also taught the company how to mass-produce aircraft.

The first Harvard Mk. Is were sent overseas in December, making them the first American aircraft to be delivered to the UK. The Hudsons would be delivered in February of 1939.

The company was still ignored however as one which could produce a high-performance fighter...

NAA's first attempt at a pursuit aircraft © USAAF
MInes Field
MInes Field
North American NA-40 prototype NX14221
North American NA-40 prototype NX14221
North American NA-40 prototype NX14221 © Ray Wagner - San Diego Air and Space Museum
NAA NA-62 prototype 40-2165
NAA NA-62 prototype 40-2165
NAA NA-62 prototype 40-2165 © USAAF
NAA P-64
NAA P-64
The P-64 was the designation assigned by the USAAC to the NAA NA-68 fighter, an upgraded variant of the NA-50 developed during the late 1930s. Seven NA-50s were purchased by the Peruvian Air Force, which nicknamed it Torito ("Little Bull") © USAAF


On January 5th, 1939, the British Air Ministry delivered their “High Speed Single Seat Fighter Specification, F. 18/39. Operational Requirement OR.73 for the Replacement of Spitfire and Hurricane” to NAA. (7)

The report specified:

"Requirements: To meet Operational Requirement OR.73, a high speed single-seater fighter capable of operating in any part of the world is required as a replacement for the Spitfire and Hurricane. The outstanding requirement is to obtain the greatest possible superiority in maximum speed over the contemporary bomber. There may be advantages to be obtained from a twin-engined design which would be acceptable, provided the performance was superior to that which could be obtained from a single engine aeroplane. The AUW [all-up weight] of an aircraft built to this specification is 12,000lb.

The speed at 15,000ft must not be less than 400mph with the 37.V/12 engine and it is essential that the engine selected should be equipped with a two-speed supercharger so that speeds at altitudes below 15,000ft may be as high as possible. The normal fuel load is to be sufficient for 15 minutes of flying at maximum engine speed for take-off conditions, plus two hours at the most economical cruising air speed at 15,000ft, plus 15 minutes at 15,000ft at maximum engine speed for level flight at 15,000ft, plus 30% reserve. Engine: The aircraft shall be designed to accommodate a 37.V/12 engine but a Rolls-Royce RM.2SM engine is to be installed in the first instance.

The design requirements of AP.970 ("Handbook of Strength Calculations"), any corrections thereto and all current ADMs applicable to TV class aeroplanes are to be satisfied at 1.1 times the AUW in the fully loaded condition at factors given in the fully loaded condition for “other than experimental aeroplanes.”

Two cannon guns in each wing set to fire along line of sight, with at least one drum of 60 rounds of ammunition per gun. Provision shall be made for the installation of a reflector gunsight and a bead sight, and a Cine Camera gun, mounted internally. Protection for the pilot against armour piercing 0.303-in. ammunition is required to cover a forward cone with an angle of 20 degrees to the thrust line of the aircraft."

Lockheed's XP-38 prototype (37-457) first flew on January 27th, 1939 but was lost in a crash on February 11th, caused by pilot error. Although not officially in any competition, the XP-40 was at that time projected to be the best US fighter available. The first prototype placed the glycol coolant radiator in an underbelly position on the fighter, just aft of the wing's trailing edge. Lt. Benjamin S. Kelsey flew this prototype some 300 miles in 57 minutes, approximately 315 miles per hour (507 km/h). Although the top speed was somewhat disappointing for Kelsey, he remained a true fan of the Allison V-12. He ordered the aircraft to be evaluated in a NACA wind tunnel to identify solutions for better aerodynamic qualities.

NAA meanwhile was still desperate for any order they could get: on February 9th, they opened the NA-51 program, where they were able to produce 47 O-47Bs. These were armed versions of the O-47 observation plane. Only 24 were destined for the USAAC. Luckily for NAA, orders for all kinds of variants of the Harvard were still coming in that year: 50 NA-50s destined for Peru, 50 NA-56s for China, 230 NA-57s for France, 251 BT-14s for the USAAC, 30 NA-16-Es for Canada, 230 NA-64s for France, all on top of the already existing 600 Harvard order for the UK.

In March of 1939, the Army Air Corps issued a specification for a medium bomber, under Request for Proposal CP 39-640. The bomber had to be able to carry a payload of 2,400 lb (1,100 kg) over a total distance of 1,200 mi (1,900 km) at a speed of 300 mph (480 km/h).

NAA, always on the lookout for extra contracts, dusted off its NA-40B design and began to develop the NA-62, which would eventually lead to their successful B-25 design. Contenders for this proposal were Douglas with the B-23, North American with the NA-40B and Glenn L. Martin with their 179 (B-26).

Three other Requests for Proposals followed that same month:

  • CP39-780 for a Multi-Place Pursuit, Specification C-618
  • CP39-775 for a Twin-Engine Interceptor Pursuit, Specification C-615
  • CP39-770 for a Single-Place Pursuit Specification C-616

The Multi-Place Pursuit request was quickly dropped. For CP39-775 Kelsey later explained that he and Saville drew up the specification using the word "interceptor" as a way to bypass the inflexible Army Air Corps requirement for pursuit aircraft to carry no more than 500 lb (230 kg) of armament including ammunition, and to bypass the USAAC restriction of single-seat aircraft to one engine.

Main goal of these requests was to produce a fighter with a better performance than the current P-39 and P-40 designs.

That same month, NAA cancelled all work on the NA-53 project. They realized that the Pratt & Whitney R-1830 design was no match for both the P-39 and P-40.

The USAAC-MD further specified that both remaining proposals were permited to be designed around in-line engines. This made NAA go back to the before mentioned Millikan report.

The funds which were freed from the abandoned NA-53 project went into the design of NAA's more advanced pursuit aircraft, the P-509.(8)

Lockheed's Model 322 and the Grumman XP-50 competed for the CP39-775 project. The Model 322, a design very similar to the XP-38, won the competiion. The primary difference between the 322 and XP-38 were the upgraded Allison V-1710-27/29 (F2) engines. 

The USAAC was still looking to add more pursuit aircraft to their inventory. In April, they made the largest single military aviation order in the US since WWI, when they put down an order for 524 P-40s. Further orders were placed with Bell for their P-39, which also attracted French interest. Due to the large USAAC and French orders for the P-40, the existing Curtiss production facilities became overstretched and could not keep production numbers up with the demand.

In late May, early June, the Kilner Board convened to prepare a comprehensive 5-year program for research and development for the Air Corps. The main goal was to catch up with the superior foreign aircraft.

In July, the British Air Mission (H.C.B. Thomas of the Royal Aircraft Establishment (RAE) and Charles Luttman) engaged with NAA to see what could be done to produce extra fighter aircraft for the RAF.

Edgar Schmued had already put up several concepts for the CP39-770 Proposal earlier that year. It is believed that during this meeting, NAA opted to the British for the first time, their own fighter design.

As luck would have it for NAA, Gen. George Brett, Chief, USAAC-MD, also ordered an increased budget for the new single-seat, single-engine, pursuit interceptor. What was more is that the Proposal did no langer call for an actual prototype to be built, as the design alone could now be declared winner of the competition.

For the single-engine pursuit competition, the Seversky AP-4J and AP-10, the Douglas DS-312A, the Bell XP-45 and Curtiss XP-46 were submitted for evaluation. Although the XP-46 finished third behind the AP-4J and DS-312A, General George Brett recommended development of the XP-46 on September 1st, 1939. He also assigned it top priority for testing when it arrived at Wright Field.

The biggest reason for this was that Curtiss claimed that it would reach "410mph at 15,000ft when fully loaded, including armor plate, armament and self-sealing tanks".

The Allison-powered, turbo-supercharged XP-39 was no match for any European single-engined fighter currently in service.

As for Seversky, the Seversky Aircraft Corporation had last over half a billion dollars and Seversky was forced out of the company. He was replaced as president of the company by W. Wallace Kellett and the company was renamed Republic Aviation Corporation. Their AP-4 eventually went into production as the P-43 Lancer, of which 272 were produced (108 of those went to China with the American Volunteer Group (AVG) or Flying Tigers, to fight against the Japanese).

Both Republic and Curtiss were asked to submit lightweight versions of their designs, which resulted in the XP-46 and XP-47 respectively, but neither design showed significant improvements over the P-40, so neither was ever produced.

Despite all of the above efforts by the US aviation industry, they still failed to produce any single-engined fighter that would equal or top the German or British designs. At the time, the US was also unaware of the newest Japanese addition to the fighter realm: the Mitsubishi A6M Zero, which first flew on April 1st.

© New York Times

A bold proposal

The situation in Europe started escalating quickly when Germany invaded Poland on September 1st, 1939. Following the aggressive act by Hitler, both Britain and France honored their prewar agreements to declare war on Germany. The United States declared its neutrality, but did enforce an arms embargo. The US government quickly modified the original bill to allow foreign powers to purchase arms for cash only if they were taken from the country in their own vessels. A condition was that the US This was the so-called "cash and carry" law. With this principle, the British and French Governments would actually be funding the development of new types for the USAAC.

Although Britain and France declared war on Germany, there would be little or no fighting until Germany would start their Blitzkrieg across Europe. This period of "political hostilities" is known as "The Phoney War".

The USAAC expansion program was still lagging... the Air Corps had only 23 B-17s in its inventory, with about 100 more on order by then. They also made sure not to put all of their eggs in one basket as far as orders for long-range bombers were concerned. Hap Arnold was tasked with finding a second option, which he found at the Consolidated Aircraft Company, who were beginning work on their XB-24.

Within days of the invasion of Poland, the French and British were back shopping for everything they could get their hands on.

Between January 1938 and the outbreak of the war, only the following aircraft types had been released for foreign governments (many of those being obsolete designs):

Edgar Schmued © SDASM Archives

Edgar Schmued was born in Germany in 1899. He was fond of mechanics and aviation and started working as a student in a small engine factory in Germany.

At a later age he started studying aeronautics and enlisted in the Austro-Hungarian Flying Service at the outbreak of WW1.


After the war his brothers immigrated to a German community in Sao Paulo, Brazil.  Edgar would follow their example in 1925 and found a job with General Motors automotive division. GM held stock in several other companies at that time, including Fokker Aircraft Company.

They were impressed by Schmued and helped him to immigrate into the US in 1929, where he held a position at Fokker in Teterboro, NJ.


In 1929, NAA took over the Fokker Aircraft Corporation and Schmued, who worked for GM as a field service manager in Brazil, moved to the USA to work with aircraft - something he had always wanted to do, with his training as a mechanical engineer.

Fokker soon became General Aviation, but the Airmail Act of 1934 forced airmail carriers to rid themselves of holdings that controlled aircraft construction. This meant that GM had to get rid  of NAA which, in  turn, took  over General Aircraft. Kindelberger had come from Douglas Aircraft a year earlier, and was now president of the 'new' NAA. 'Dutch' moved the company west and asked Schmued to join him, but Schmued's wife did  not want to  move to  California so  the designer joined  the firm of Bellanca - a decision he soon regretted.


However, 'Dutch'  had  kept the job  offer open, and Schmued and his family finally moved west in 1936. However, just 100 miles from Los Angeles, the family was involved in a serious car accident which killed Schmued's wife and left the designer seriously injured - it took him until February 1936 to recover.

Whilst working as Chief Engineer, Atwood had looked into the 1939 Meredith report regarding energy recovery from airplane radiators. With this in mind, he began to think about how this might be applied to the P-40 design.
He figured that, despite the extra plumbing and added weight, it would still be favorable to put the radiator in the fuselage, with only the duct opening exposed. The P-40 had the cooling system up front underneath the engine, so in order to balance the plane, Curtiss had to move the pilot more towards the rear, thus somewhat compromising his view and limiting fuselage space.

In his new position as VP, he had the responsibility to handle contract administrations. Since both offices were relatively close to each other in New York, Atwood regularly visited the BPC to negotiate contracts, prices, spare parts, equipment and support services for previous contracts such as the Harvard.


The BPC, in addition to Sir Henry, also employed H.C.B. "Tommy" Thomas, who was a senior technical man there. Atwood started pitching the coolant drag reducing theories to Thomas and both men visited each other regularly. Atwood also showed the report NA-1592, along with the drawings.

After a couple of meetings, Thomas had made some inquiries with Farnborough on the subject and Atwood discussed the concept with Edgar Schmued, NAA's design supervisor.


In early April, Atwood was invited to Sir Henry's office and was advised approximately as follows:

  • They had decided to accept NAA's proposal;
  • Atwood should prepare a letter contract for his signature, providing the purchase of 400 aircraft;
  • the letter should contain a schedule and a not-to-exceed price per aircraft;
  • that the British-supplied equipment, including engines would be specified;
  • a definitive contract would be negotiated on the basis of this letter.

Sir Henry also told Atwood that, since NAA had never produced a pure fighter, he considered it very desirable that they had some Curtiss P-40 data as a helpful guide (he further specified wind tunnel reports and flight test reports). (16)

As told by Lee Atwood himself: “I made several trips to New York from January to April and stayed at the Essex House most of the time. I received some assistance from the General Motors offices at 1775 Broadway, not far from the Essex House. Also, I was assisted from time to time by R. L. Burla of the NAA staff and L. T. Taylor, then based in Washington, DC for the company. A. T. Burton who had been stationed in England for the Harvard program also assisted. The Chadbourne law firm gave me legal assistance, mainly through Ralph Ray, a partner in the firm.


I made it clear that we had no design, but that if authorised to proceed, we would design and build the aircraft in accordance with the representations I had made to the BDPC. These conversations went on until about the last week in March or the first week in April,

when apparently affirmative recommendations were made to Sir Henry Self. At that time he called me in and discussed the project and asked me for a definite proposal. He made a reservation, however, and took note of the fact that we had not ever designed an actual fighter plane. He asked me if I thought I could get copies of the wind-tunnel tests and flight tests of the P-40 airplane.


He said if I could, it would increase their confidence in our ability to move forward in a timely way. I told him I would try, and that night I took a train to Buffalo where I called upon Mr Burdett Wright who was general manager of the Curtiss division at Buffalo. After negotiating with him for most of the day, I arranged to purchase copies of the wind-tunnel tests and the flight test report for the sum of $56,000, which would cover the out-of-pocket expenses and some proportion of the cost of the tests.


I went back to New York and indicated to Sir Henry that I had been able to secure the data and presented him with a draft of the letter contract, which called for the production of 320 NA-73 aircraft equipped with an Allison engine and certain armaments to be furnished by the British, and an airframe to be designed and built by NAA – the total cost to the British government excluding engine, armaments, etc., was not to exceed $40,000 per airplane.


Although some technical work was by then being done in Los Angeles, we had not at this time presented the British Purchasing Commission [BDPC] with drawings or specifications of any kind except for free-hand sketches I had used to demonstrate the concept in informal conversations, and a letter contract was the sole document available. Sir Henry Self executed this document, after having it edited by his legal staff, and with this instrument the new NAA project got under way.”


Atwood went to Buffalo, New York, to  collect the papers and  data. In correspondence with Sir Henry Self, Atwood would write on May  1st, 1940: “'We have reached  an  extremely  satisfactory  agreement with  the Curtiss Aeroplane Company  of Buffalo  whereby they  are furnishing  to  us data covering  a comprehensive series of wind tunnel, cooling and performance tests of a similar airplane, which  data will assist us in  the design  and  manufacture of these airplanes”.

At last, all of NAA's hard work had prevailed and they were given a change to prove themselves with an official fighter design. It certainly helped in the negotiations that the company had shown the British they could handle mass production well with their Harvard trainer.

As mentioned by Atwood, he made the trip to Buffalo, where Curtiss was located, that very same evening after his meeting with the BPC. At Curtiss, he met with Burdette Wright, the GM of the Curtiss Airplane Company. With the reported aid of Col. Ben Kelsey of Materiel Command, Curtiss were reasonable enough to sell the data. Contrary to what many have written, this was the data of Curtiss' XP-46, not the P-40. Burdette asked Don Berlin, who had spent the last years developing the more advanced XP-46 of selling the data to NAA. Berlin agreed, so the XP-46 data was sold to NAA on April 10th.

In summary: after months of lobbying from the BPC to build P-40s at NAA, the company was able to secure an agreement for its own fighter design in just two days: Atwood met with the BPC on April 9th (the same day Germany started the invasion of Denmark and Norway), made the overnight trip to Curtiss where he secured the XP-46 data on April 10th (the same day Bell secured a contract with the BPC for their P-39), redacted the letter contract and returned to New York to meet with the BPC signed the go-ahead letter by the end of the day!


NAA purchased wind-tunnel data and test reports of the Curtiss Model 81, which was a design (following the P-40) that was entered by the Curtiss -Wright Corporation in 1939 for Request for Proposal CP-39-770. Although it came in third in this competition, after the invasion of Poland, the Air Corps dropped the first two in favor of the Model 81.

This was mainly due to the Air Corps' quest to find an aircraft capable of speeds as fast as the Bf-109.

In September of 1939, the Air Corps had ordered two prototypes from Curtiss, which were designated XP-46, with serials numbers 40-3053 and 40-3054 being attributed. Following a request from the Army's Materiel Division, a full-scale model of the XP-46 was prepared for tests in NACA's wind tunnel.

The XP-46 was a single-engine, low-wing aircraft and was slightly smaller than the P-40. It featured a wide landing gear track which retracted inward. The planned armament included two .50 in (12.7 mm) machine guns in the nose below the cylinder banks, firing through the propeller and no less than eight .30 in (7.62 mm) wing-mounted guns. This made it the most heavily-armed American built fighter up to that time. Planned maximum speed fully loaded was to be 410mph at 15,000ft. Engine of choice was the newly-developed Allison V-1710-39 (F3R) twelve-cylinder liquid-cooled inline example, producing 1150 hp.

About a month after their initial order, the USAAC modified their requirements and called for the addition of self-sealing fuel tanks and about 65 lb (29 kg) of armor. The added weights of these changes would adversely affect performance of the XP-46.

The scope of the test program at NACA was to determine the optimal configuration for maximum speed, optimize engine-cooling concepts and to  examine and improve aerodynamic characteristics. It was found that a long-fuselage-nose configuration showed better maximum lift capability.  The wind-tunnel model was tested with three different belly scoops. The one originating from Curtiss proved to work insufficiently. NACA tried to improve the design, but work did not progress sufficiently to provipe proper cooling for the engine.

In order to quickly get something in the air, the second prototype was delivered without the armament and radio equipment and was designeted XP-46A. The first example was completed with all of the equipment and armament.

The second prototype, XP-46A, was actually the first to be test flown, on February 15th, 1941. Even devoid of all military equipment, the XP-46A was just barely able to achieve the projected speed of 410mph at 12,200ft.

When the first prototype was test flown on September 29th, 1941, the fully equipped example was much slower and only attained a top speed of 355mph at 12,200 ft.

During test flights, it would also prove that the belly scoop system failed to cool its systems properly.


The Air Corps found these numbers not significant enough to warrant a P-46 production run and the program was subsequently cancelled. When the P-40 was given the same engine as the XP-46, it even proved to be faster. Because the P-40 production lines were already up and running and because it also sported the necessary armament and self-sealing fuel tanks, the Air Corps decided not to disrupt the Curtiss production lines at this critical period. 

The upgraded P-40 was designated the P-40D and became known as the Kittyhawk in British service.

Because NAA had been granted access to the data of the XP-46, several sources claim the Mustang prototype to be a copy of the XP-46, taking its belly-scoop design. Neither Curtiss, nor NACA were able to get the XP-46 fuselage cooling design working adequately.  Also, Lee Atwood had been pitching the idea of the Meredith study to BPC's Thomas as early as January, at which time NAA did not have the data from Curtiss. Additionally, the first Curtiss prototype flew on February 15th, 1941, three and a half months after the first flight of the NA-73X.

NAA also never made a secret of the purchased material. Chief aerodynamicist Ed Horkey  examined the data and  found  it virtually  useless. Schmued  would  later claim he never even saw the Curtiss data.

It is very unlikely that any information obtained from the XP-46 would have been of any practical assistance. Comparing both the XP-46 and the later NA-73X externally shows that the only similarity was a radiator set below the fuselage under the cockpit. (17)

On April 11th, Sir Henry Self signed a letter of intent to purchase 400 NA-50B fighters following NAA Spec. 1592 as amended. Note that the letter of intent stipulated "NA-50B". Why this was the case is still unknown as the new design did bore any resemblance to the previous NA-50 model. It is believed the "B" addition stands for "Britain".

Upon Kindelberger’s return to Inglewood, he held meetings with vice president Lee Atwood, chief engineer Ray Rice and chief of aircraft design Edgar Schmued. The main players at NAA involved in the design program included Kindelberger, Atwood, Schmued, Rice, chief aerodynamicist Ed Horkey and key aeronautical engineers Art Chester, John Young, Marc W. Malsby, J. Stan Smithson and Larry Waite.

Shortly after these events, shop order SC-1050 came trough at NAA for the construction of a mock-up of their design. As Schmued recalled: "in three days we had it built, using paper, plaster of paris, whatever was suitable". (18)

The model design team was given a deadline of April 17th.

Schmued began detail design on the X73 on April 21st. He was given a completely blank checkbook by Kindelberger and he could hand-pick his design team as he pleased. Schmued later stated: "I made the best of this and picked the best people I could find. I started with eight and had about 14 or 15 two weeks after we started – and planned on growth up to 49 people, then drop down to 10–12. As we made this schedule, we forecast that we needed 100 days to design and build the airplane. Then, as you might say, “hell broke loose.” We made a very careful time-study of this project. Each man who led a group – like wing group, fuselage group, powerplant, landing gear – was called in and made his own estimate of the time he thought he needed to get his drawings and data out to the Experimental Department."

The ball started rolling almost instantly after Kindelberger sent a telegram back to his designers at Inglewood on April 24th. Chief of engineering Raymond Rice and assistant chief design engineer Edgar Schmued both immediately set their respective teams to work, despite it being a Saturday.

The new design became known as NAA charge number NA-73 (the next available number), with the prototype being designated NA-73X (X for experimental).

The design teams continued working through the night on general arrangement drawings and a preliminary weight study, which ultimately resulted in an  NAA report titled “NAA 1620 Detail Specifications for Model NA-73 (a common misconception is that the report stipulated the handoff of the 4th and 10th production aircraft to the USAAF as XP-51s, but this was only added two years later in the revised edition of the report). 

It was delivered in time for Kindelberger to view the results by 10 am on April 25th.


© GALCIT Report 286-A

The following is a letter from NAA's Lee Atwood to Sir Henry Self of the Anglo-French Purchasing Commission.The letter was dated May 1st, 1940:


“In accordance with our understanding, we are proceeding with the design of a single-seat fighter airplane, our Model NA-73, incorporating an Allison engine and fitted with provisions for equipment and armament as detailed more completely hereunder.

We have reached an extremely satisfactory agreement with the Curtiss Aeroplane Company of Buffalo wherein they are furnishing to us data covering a comprehensive series of wind tunnel, cooling, and performance tests of a similar air­ plane, which data will assist us in the design and manufacture of these airplanes. We have also received release from the United States Army for the manufacture and export of these airplanes and wish to assure you that all arrangements are entirely satisfactory.

We are prepared to construct and deliver to you 320 of these airplanes before 30 September 1941, and guarantee to effect deliveries in accordance with the following delivery schedule:




















Spares (*)










(*) Equivalent aircraft

We further offer to continue the manufacture of these planes at the rate of 50 airplanes per month until at least the end of the year 1941, should you desire to incorporate and exercise an option for these additional airplanes prior to 30 April 1941.


We have constructed a mockup and have completed the initial phase of the detail design and are submitting to you herewith certain data and information regarding the characteristics of the airplane. You will note that we have provided for armor protection for the pilot and a sealing arrangement for the fuel tanks.

Provisions are being made for the installation for four .50 caliber machine guns, two of which are in the fuselage and the other two in the wing. As a normal load we are specifying 200 rounds of ammunition per .50 caliber gun, but are making additional provisions for more ammunition as a special load. Provisions are being made for four British Type 303 machine guns with ammunition boxes to accommodate 500 rounds of ammunition per gun as normal load.

'Strictly for comparative purposes, we are including the results of a study showing the difference in size and performance between the airplane offered and one which might be offered with a minimum armament and without protective armor, but is otherwise the same. It will be noted that the high speed in this condition is 400 mph with a wing area of 190 sq ft.

With a full complement of armament and armor plate protection front and rear, the weight is increased from 6450 lb to 7765 lb and the wing area is increased from 190 sq ft to 230 sq ft in order to maintain the same landing speed. The resulting performance is materially reduced and high speed is 384 mph under the same conditions


The speeds quoted above are based on a power of 1030 hp at 16,000 ft altitude, using 90 octane fuel. Since we do not have precise and final information on the power rating of the engines to be furnished for these airplanes, this rating is still somewhat of an estimate. We believe the Anglo- French Commission has, or will shortly have, accurate information on this matter. When we receive the exact figures, the performance guarantees will be arithmetically adjusted accordingly.


The general provisions for armament have been discussed with Air Commodore Baker and Mr. Thomas and it is believed that the arrangement offered is the most practical possible at this time, consistent with the general instructions we have received. It is possible to increase the fire power through the installation of additional guns if absolutely necessary, but the performance will suffer a proportionate loss. We  feel  there  will be no difficulty in making any changes or modifications which you may feel are essential or desirable, and are prepared to co-operate with your technical staff to the fullest extent. We do feel, however, that the design as presented is close to an optimum condition, all things considered. Details of equipment and installation are yet to be covered, but our previous experience with Harvard aircraft, which incorporate much British equipment, leads us to believe that we will have no difficulty whatever in arriving at satisfactory agreements in all these matters.


We have made a careful estimate of the price, including sufficient structural tests to guarantee the structural integrity of all parts, wind tunnel testing and flight testing. We have included a price breakdown, separating and pricing all items of equipment to be installed and supposedly furnished by us. We have not considered the price of engine, propeller, radio, oxygen, machine guns or other items of armament or military equipment, and it is assumed that these items will be furnished to us free of charge for installation in the airplanes. The price summary for airplanes, exclusive of crating or transportation, but covering all other charges is as follows:

A) Powerplants, engine accessories

$ 983.95

B) Instruments

$ 1,787.35

C) Electrical Equipment

$ 890.75

D) Miscellaneous Equipment

$ 528.40

E) Radio Equipment

Customer Furnished

F) Armament

Customer Furnished

Total contractor furnished equipment

$ 4,190.45

Base airplane

$ 33,400.00

Total per airplane

$ 37,590.45

Total for 320 airplanes

$ 12,028,944.00

Spare parts (20%)

$ 2,405,788.80

Crate per airplane at $675 total

$ 216,000.00

Crating for spare parts

$ 96,231.35

Total Contract Amount

$ 14,746,964.35

Within 60 days after the contract has been executed we will furnish a complete percentage breakdown and a recommended list of spare parts to approximate 20% of the contract prices of the airplanes. Spares will be delivered in accordance with the delivery schedule attached hereto provided a spare parts list is approved and agreed upon within 60 days after submission of such a proposed list by us.

We are prepared to proceed immediately upon receipt of a letter from you accepting this proposal and receipt of down-payment. We desire a down-payment of 10% of the contract amount upon approval of this proposal and a subsequent monthly payment of 2.5% of the contract amount each month until 25% of the contract has been paid.

Details of final payments and acceptance will be as mutually agreed upon and in general accordance with our previous contracts with the British Government. We feel there will be no difficulty in the preparation of the final contract at your convenience inasmuch as we have reached agreements with your staff concerning all principal points involved in a contract of this type.

'The prices quoted above are intended to include all normal and reasonable modifications and changes which you may require, provided that such changes are agreed upon within three months of the date of the agreement and pro­ vided there is no considerable additional expense to us as might be involved in the purchase of additional material or equipment. Changes initiated after this time may involve a delay in delivery or a cost increase.

'May we request that you give this matter your early attention as we are prepared to proceed on receipt of a letter of approval from you and receipt of down-payment as requested above. We will consider the date of receipt of this payment as the date of the contract.

'If there are any matters not properly covered in this letter or the enclosed data and it is necessary to withhold the letter until such matters are clarified, we will greatly appreciate it if you will notify us of these matters by telegram or telephone at our expense in order that there will be no delay.”

The contract specified initial prototype delivery in January of 1941 and completion of 320 airframes by September 30th, 1941.

Contrary to what many sources state: there was never a mandate or a promise to complete and fly a prototype within 120 days!

As promised, the drawings were delivered to Kindelberger’s office and the president of NAA was very happy with what he saw. The Schmued-produced drawings presented a very sleek, low-wing monoplane with every effort incorporated to keep drag to a minimum. The  design was simple yet functional and Kindelberger felt confident that the British would be pleased.

Schmued held a design meeting with project engineer Ken Bowen on April 29th, between NAA and the RAF. In the report which Bowen made after the meeting, 14 points of concern were stipulated, including the possible impact on the schedule if work were to continu with the P-509 NACA wing.

At the request of the AFPB, Lee Atwood submitted three possible armament configurations for their design, which included a 20mm cannon variant and also a mixture of .50cal and .30 cal Browning machine guns.

NAA airmailed the British delegation drawings of a design concept on May 3rd, 1940.

At that time, the USAAC reserved for itself the right to block any foreign aircraft sales that it regarded as not in the Army's interest, for whatever reason.

As mentioned earlier, NAA's first letter of intent stipulated model number NA-50B. The NA-50 was originally intented for export to South America and featured a radial engine. When Gen. Echols of Materiel Command received NAA's request for export release, he noticed that the engine was different from what was stipulated.

It was Echol's superior, Gen. George Brett, at the USAAC-MD HQ in Washington, D.C., that issued clearance on the Foreign Release Agreement with respect to “forbidden technology for Export guidelines,”. The document would be signed by the AFPB, the USAAC, and NAA two weeks later, on May 4th.

This was the first attempt by Echols to hinder NAA, because the company had foiled his plans with the P-40 and P-46. This suspicion is further augmented because the P-39, P-40, P-46 and Lockheel Model 222 had already received an export approval to Europe.

Because Dutch Kindelberger and his company had rejected the USAAC-MD’s request to build P-40s under license, and thus eliminated Echol's plans to aid Curtiss in getting their P-46 ready, Echol's set out on a long-lasting personal mission to slow down NAA's plans of getting their fighter approved.


Anyway, the USAAC had no interest in the aircraft at that time. Because of the signed "Foreign Release Agreement", NAA was now allowed to write formal contracts with the BPC from then on.

It was thanks to First Lieutenant Benjamin S. Kelsey, head of the Army Air Corps Pursuit Project Office, that a condition was added to the foreign release contract: two examples of the initial production batch for Brittain were to be turned over to the USAAC for testing, free of charge. In other words, this meant that the British would buy the US Army two aircraft for which at that time it did not have the funds available to purchase them themselves.

It was agreed that the USAAC would receive the fourth and tenth production airplanes.

Following the outbreak of the war and the resulting increase in need for additional aircraft from Britain and France, manufacturers were permitted to export other types. First additions were the Douglas DB-280 (B-18A), Douglas DB-7A (A-20), Fairchild M-52 (PT-19) and the Curtiss 81A (P-40), but the latter only without the closely guarded supercharger.

France had made new contracts with Curtiss which resulted in the completion in 1940 of 420 Hawk 75A and 259 H81A (export version of the Army's P-40) fighters. Most of these would however not be finished in time to help France.

On September 5th, the Air Corps ordered the NA-62 into production as the RB-25, along with the other new Air Corps medium bomber, the Martin B-26 Marauder "off the drawing board". The USAAC contract number AC-13258 called for the production of 184 medium-class twin-engine bombers: 24 RB-25s, 40 RB-25As and 120 RB-25Bs.

With this contract, NAA finally landed its first huge US Government contract which guarenteed production continuity. The "R" prefix (Reconnaissance) was dropped soon and the production aircraft became known as the B-25.

On September 17th, Russia invades Poland from the East. The German and Russian foreign ministers would meet on September 28th to finalize the division of Poland.

Britain's need for aircraft and engines was so significant that they established a seperate headquarters on 15 Broad Street, in Manhattan, New York, on November 9th. This office, which was tasked with aviation purchases in the US, was named the British Purchasing Commission (BPC), headed by Arthur Purvis. A sub division called the British Air Commission was created to concentrate on aircraft and other aviation-related materiel. The BAC was headed by Sir Henry Self. (9)

It was a significant step towards the signing of hundreds of orders with US companies.

Most pressing issue was the acquisition of fighter aircraft. Not for Fighter Command, for there was no American production match for a Spitfire, but for the empire's valuable colonies in the Middle and Far East, threatened by Germany's Axis partners.

The French had already put a substantial order for the Curtiss Model 81 (P-40). The British were well aware of the backlog in production at Curtiss. The company had been very slow in series production of the P-40, the initial aircraft delivery only being made in the mid-1940s.  In looking for solutions, the BPC began looking for companies willing to build the P-40 under license.

Sir Henry Self was already acquainted with NAA, as he was involved in the Harvard purchase the previous year. It was not long before he and other BPC representatives approached NAA with the proposal of helping them out by building P-40s.

Lt. Kelsey and Brig. Gen. Echols of Materiel Command regretted the push to build the P-40. When Hap Arnold came to Wright Field during this period, Kelsey recalls. "Echols told him that if the US could just hold off building these P-40s for a while and not try to increase production, we could rush through and substitute the P-46 for the P-40 in the build-up. Then we'd have the airplane we should have and not the one that was locked, essentially, into the 1936 P-36 procurement."

A couple of days later, Arnold replied: 'We're committed to a training program with a great many pilots and to a deployment program that involves the creation and commissioning of new groups. We just can't afford to lose the P-40s.'" (10)

Although the XP-46 placed third on the CP-39-770 Request For Proposal, Materiel Command and several other instances still favored it above anything else. The plan was for it to be the improved fighter for the Air Corps and its development would be paid by the foreign sales of the P-40.

Kindelberger, Atwood and Rice had already envisioned what it would take to builld other aircraft, such as the P-40, under license. The consensus was that it would take far too long to retool everything to efficiently build the meanwhile obsolete P-40 design. As a result, at NAA ideas were brewing to present a the Company Sponsored Pursuit (P-509) as an alternative.

On November 30th, Russia invades Finland. This is the start of the so-called Winter War.

Edgar Schmued and his team worked on several drawings for the SC-46/P-500 Ranger Powered Pursuit, starting December 18th, 1939. By the end of January, 1940, those drawings included the inboard profile, cowling, the wings, landing gear, and a set of three-view drawings. (11)

When Kindelberger and Atwood traveled to New York City to meet with the Anglo-French Purchasing Board (AFPB) on February 1st and 2nd, 1940, they took those designs with them. Unfortunately, both the British and French quickly recognized that the proposed design would not be able to compete with the Luftwaffe frontline fighters.

When the meetings came to an end, the AFPB formally requested that NAA build P-40s instead, a request that was supported by Lt. Col. Oliver Echols of Materiel Command.

Not to be deterred, Schmued and his team continued to develop and refine their fighter design, which resulted in the P-509. The upgraded design illustrated a streamlined lower inlet/radiator cowl just aft of the wing. This would house the radiator and oil cooler and also showed the lengthened exhaust fairing and exit.

Meanwhile, NAA executives deliberated over the options of ignoring the USAAC-MD versus making one more try to convince the AFPB. As Schmued recalled: "One afternoon, “Dutch” Kindelberger came into my office and asked, “Ed, do we want to build P-40s here?” I replied, “Well, ‘Dutch,’ don’t let us build an obsolete airplane, let’s build a new one. We can design and build a better one.” And that’s exactly what he wanted to hear. So he said: “Ed, I’m going to Great Britain in about two weeks and I need an inboard profile, three-view drawing, performance estimates, weight estimate, specifications and some detail drawings on the gun installation to take along. Then I would like to sell that new airplane that you developed.” He said the rules for design were simple. Make it the fastest airplane you can and build it around a man that is 5ft 10in tall and weighs 140lb. It should have two 20mm cannon in each wing and it should meet all the design requirements of the USAAC." (12)

Schmued said that he looked around to find an engineer or somebody in the organisation that met the weight and height requirements set by Kindelberger. They found a man fitting the description in their Engineering Department. It was Arthur C. ‘Art’ Chester, who would later become project manager of Mustang engine installations. Therefore the airframe and cockpit were designed around him.

He had also prepared the three-view drawings and reports of the new fighter.

The P-509 is the first known example of NAA’s introduction of the “Meredith Effect” ramjet exhaust cooling approach. The design also retained the same airfoil as the P-500, which was the NACA 2516-34. The only difference was that it had a longer fuselage and larger empennage. The very first drawing of the P-509-1 was in NAA's “Preliminary Design Data – Single Seat Pursuit (Allison Engine) – General” report, which was completed around December 18th, 1939.

In this report, the P-509 was assigned the slightly less powerful Allison V-1710-35 engine, and lacked armor and self-sealing wing fuel tanks.


In the first month of 1940, the BPC continued its shopping spree across the US. According to the New York times, on January 21st, "England has spent an average of $ 9,000,000 a week in the United States for military supplies since the British Purchasing Commission was established here in mid-November".

As mentioned in Report No. 1592 in early March, NAA continued their work on their fighter design. The following changes to the design were made:

  1. The addition of self-sealing tanks and armor plating (which also increased the gross weight from 6,455lb to 6,540lb)
  2. The wing area was slightly enlarged
  3. The main-wheel tire diameters were increased from 27in. to 30in.
  4. The newer Allison V-1710-39/F3 was used, resulting in a slight increase in power.
  5. The two .50cal AN/M2 Browning machine guns were moved to the wing root, instead of under the cowling., but were still designed to fire through the propeller arc.
  6. A specification that the airfoil would be a “NACA series airfoil developed for high speed performance”. (13)

By March 10th, several releases of the drawings were made, consisting of different armament layouts, including one with 20mm cannons.

On March 14th, British and French officials meeting with the president’s Liaison Committee insisted that they had to have the P-38 and P-39, as well as the P-46, in 1941. Air Corps leaders had resisted these requests before, but now they realized that the 789 pursuits the Army had ordered in 1939 lacked armor, selfsealing tanks and sufficient firepower. Letting the Allies buy into the fighter program was a way of funding these crucial modernizations. (14)

On March 25th, the government’s new Release of Aircraft Policy was completed: from now on, the Allies would also be allowed to buy certain modern types, providing that their manufacturers would offer a more advanced design to the Army and that the Allies would provide information about their own combat experience. (15)

Now, aircraft manufacturers were allowed to export certain "late military types" of aicraft, such as the B-17, B-24, B-25, B-26, A-20A and the P-40. At that time, because of the funding from Britain and France primarily, aircraft manufacturers geared up their new designs and developments. As Generals Arnold and Marshall bot admitted, the release of military aircraft from the US to its Allies was essential if the US was to avoid being left with obsolete equipent.

Matters in Europe quickly escalated starting April 9th, when Hitler suddenly put an end to the "Phony War" by started his Blitzkrieg across Denmark, with the intent to capture Norway. Denmark was captured in only 6 hours, Norway would fall by the end of the month.

The German invasion of Norway once again demonstrated the great tactical use of airborne operations and of air power potential. They employed about 800 tactical aircraft in the brief campaign, along with the support of some 300 transport aircraft operating between Germany, Denmark and Norway to establish air bases in a record time at strategic points in Norway.

The following day, the AFPC was told they could complete contracts for 2,440 American fighters. Orders were placed with Bell, for the Model 14 (export version of the P-39, including armor, self-sealing fuel tanks, four wing guns and a 20mm Hispano cannon in the nose). Lockheed also received its Foreign Release Agreement for their Model 322 (P-38). In exchange, Bell would design its radical Model 16 and Lockheed was required to deliver another prototype, to be designated XP-58 and delivered within 16 months, without extra costs for the US Government.

Lee Atwood had moved up from Chief Engineer to Vice President in the fall of 1939, but kept in close touch with the engineering department. Raymond Rice took over as Chief Engineer.

© NY Times

Creating A Legend

Shortly after NAA struck the deal for their NA-73X with the AFPB, Hitler advanced his armies once again and invaded the European Low Countries: the Netherlands, Belgium and Luxemburg, all neutral countries at that time.

It was another brilliantly executed plan where, once again, the Luftwaffe played a large role. Over 3,000 aircraft participated in the invasion, more than sufficient to wipe out the weak air opposition of the invaded countries and to provide additional support for the ground troops.

The Armée de l'Air tried to form a counter attack as 71 light bombers were trying to attack the pontoon bridges carrying German armor across the Meuse river. German fighters and anti-aircraft artillery accounted for over 40 of the attacking aircraft.

Elsewhere the Germans combined ingenuity with boldness. In Belgium they used gliders to place assault troops atop Fort Eben Emael, capturing that underground redoubt. In Holland, as in Norway, they employed parachute troops and airborne infantry to seize airfields. Elements of the Dutch air force challenged the troop-carrying Ju 52s, shooting down several before Bf 109s could intervene, but the Dutch resistance collapsed after German bombers destroyed the central part of Rotterdam.

The combination of fighter air dominance, Stuka dive bombing on enemy troop concentrations and installations and air transport of troops made short work over the defending forces. By the end of May, the British troops were pushed back to Dunkirk. In just 10 days between May 26th and June 4th, 338,000 British and Allied troops were carried aboard a flotilla of boats and ships from Dunkirk. It was over Dunkirk, against the RAF that the German Luftwaffe would meet a tougher opposition for the first time.

For those interested in a bit of history, feel free to read a more detailed history of the evacuation at Dunkirk in the box below.... to continue reading the history of NA-73X, simply skip the box...

Operation Dynamo

The evacuation at Dunkirk is also often referred to as the "Miracle of Dunkirk" and encompassed the rescue of Allied soldiers from the beach of Dunkirk, in northern France.

Barely one week after Hitler had ordered the invasion of France and the Low Countries, the German army had pushed the Allied forces into a corner of north-east France near the port of Dunkirk, where they were completely cut off and surrounded.


The Allied forces comprised the retreating soldiers of the British Expeditionary Force (BEF), together with their French, Canadian, and Belgian allies. Under attack from the Luftwaffe, they faced roads blocked with vehicles and a flood of refugees.


In one of the most debated decisions of the war, the Germans (the order did not originate from Hitler himself as often stated), called a halt to the advance of their panzer divisions. This is known as the "Halt Order".

The idea came from Generalobersten von Runstedt and Günther von Kluge, who suggested that  the German forces around the Dunkirk pocket should cease their advance on the port and consolidate to avoid an Allied breakout. Hitler sanctioned the order on 24 May with the support of the Oberkommando der Wehrmacht. They stopped their advance for three days, giving the Allies some breathing space and allowing them to organize the evacuation and build a defensive line.

The German pincer movement on the Allied troops was regarded by Winston Churchill, the British Prime Minister, as the largest military defeat for centuries. True, it could have cost the British the war, were it not for the German halt and the swift actions to evacuate the troops by boat.


The German thought they had the Allies trapped and that there was no way out, considering the North Sea as a barrier, while the British saw it as their escape route.


The War Office made the decision to evacuate British forces on May 25th.


Because of the shallow waters at Dunkirk, British destroyers and transports were unable to approach the beaches, so a plan was made up to have the soldiers wade out to small boats. Many soldiers stood and waited for hours in shoulder-deep cold sea water…

Sunday May 26th, 1940 - Day 1

When Calais and Boulogne fell, Dunkirk remained the only port available for the evacuation of Allied troops. The RAF No. 11 Group assigns 16 squadrons to protect the port.  Eleven Group only had about 200 serviceable aircraft, as they suffered heavy losses between May 20th and May 25th in the Battle of France in which they lost about 54 aircraft.


Vice-Admiral Ramsay’s plan envisaged the rescue of 30,000 to 45,000 troops in just two days using a fleet of destroyers and transport ships. Just before 19:00, Churchill ordered Operation Dynamo (the operation was planned in a room of the naval headquarters in Dover that once housed a dynamo) to begin.

The German army resumes its attacks on the Alllied troop positions.


The Allies had to defend a small pocket around Dunkirk (the perimeter line only measuring approximately 12 miles inland) and were under constant attack. The coastal area above and below Dunkirk were becoming increasingly equipped with German artillery. Above all this, the Luftwaffe had bombed the main docks at Dunkirk, which was the main place from which the Allied troops could have been evacuated. There were now two alternatives left: a mole on the east side of the harbor and the beache to the north of the port. Both were far from ideal as the beaches at Dunkirk were shallow. Even at high tide, a destroyer was unable to make it closer than a mile to the shore.

© British Museum

© British Museum - infantry tank Matilda I from the 1st Tank Brigade covers the retreat of the English infantry to Dunkirk

Monday May 27th, 1940 - Day 2

Since large boats could not get close enough, the small-craft section of the British Ministry of shipping telephoned all boat builders and owners around the coast, asking them to collect all boats that could navigate in shallow waters. Most of those boats were pleasure boats and private yachts. Some were commandeered, some were taken with the owner's permission and in some cases, the owners insisted on sailing over themselves.


All of the boats were taken to Ramsgate, awaiting the order to set sail for Dunkirk.


The British fought back to the Dunkirk perimeter line.


Meanwhile, the Luftwaffe was heavily bombing the town of Dunkirk and its dock installations. They also dropped leaflets over the Allied troops, showing a map of the situation. They read, in English and French: "British soldiers! Look at the map: it gives your true situation! Your troops are entirely surrounded—stop fighting! Put down your arms!" .

The first evacuation were painstakingly slow. The Mona's Isle, a Royal Navy armed boarding vessel was the first back on British shores that morning, but only managed to recover 1,420 troops.

On its return voyage to Dunkirk it was hit by shells from enemy guns at the shore and strafed by German aircraft, which resulted in the loss of 23 men and 60 wounded.

By sundown, only 7,669 men had been rescued. The process of transporting soldiers in smaller boats between the shore and the larger ships further at sea was very time consuming. Only when a narrow jetty was found which could act as a dock where larger boats could tie up, were the evacuations progressing at a higher rate.


As word of the withdrawal by sea reached German commanders, the aerial attacks on the coast were augmented and the harbor is completely destroyed. Messerschmitt fighters strafed the retreating troops and Dorniers, Heinkels and the feared Stuka's bombed them. Fighter Command was unable to provide constant air cover from dawn till dusk.  The Luftwaffe was throwing about 300 bombers and 550 fighter escorts into the fight at that stage of the war.


Small patrols between 8 and 20 fighters were dispatched to keep the intervals between flights over the French coast as brief as possible. The RAF flew 287 sorties that day, reverting automatically to on-call status for home defense after and in between missions. They accounted for seven DO 17Zs, two Ju -88s and six He 111s, but also took a heavy toll. They lost 14 Hurricanes and five Spitfires in dogfights above the beaches.

Tuesday, May 28th, 1940 - Day 3

On this day the Belgian army,  fighting on the Lys river under the command of King Leopold III, surrendered. This left a 20 m (32 km) gap in the eastern flank between the British and the sea. As a constitutional monarch, Leopold's decision to surrender without consulting the Belgian government led to his condemnation by the Belgian and French Prime Ministers. The battle-worn 3rd, 4th and 50th Divisions were sent into the line to fill the space the Belgians had held.


Meanwhile, Erwin Rommel had surrounded five divisions of the French First Army near Lille. Although completely cut off and heavily outnumbered, the French fought on for four days under General Molinié in the Siege of Lille, keeping seven German divisions from the assault on Dunkirk and saving a lot of  Allied troops.


The evacuations were coming along better as cloudy weather and black smoke from burning oil storage tanks were hampering the visibility of the German bombers. That day, 17,804 men were rescued with minimal casualties. Hundreds of small boats bravely went back and forth between the beaches and the larger vessels further away in deeper waters, often under heavy attack from the Luftwaffe and nearby enemy artillery positions.


On the defense perimeter, fighting on both flanks remained heavy, as did casualty levels.

The Luftwaffe only lost one Do 17Z that day. During various skirmishes between RAF and Luftwaffe fighters, the score was two Bf 109s against three Spitfires, three Defiants and eight Hurricanes lost.

Operation Dynamo
Operation Dynamo
Allied troops gather at the beach
© British Museum
Operation Dynamo
Operation Dynamo
The troops on the beach were under constant attack from bombers, German fighter aircraft strafing and artillery shelling.
© British Museum
Operation Dynamo
Operation Dynamo
Allied troops on the beach awaiting rescue.
© British Museum

Wednesday, May 29th, 1940 - Day 4

Another 47,310 soldiers were evacuated, although not without losses as 650 soldiers and boat crews were killed when two British destroyers (HMS Wakeful and HMS Grafton) were sunk before dawn by German torpedo boats.


Low clouds kept enemy air attacks to a minimum, but after in the afternoon the skies over Dunkirk started to clear. The Luftwaffe launched five massive raids and hit the Allied troops defending the perimeter, the ships evacuating the beaches and the beaches themselves. They destroyed 25 vessels in total and lost four He 111s and four Ju 88s, whilst the escorting Bf 109Es downed seven Spitfires and seven Hurricanes against six of their own.

The Wehrmacht High Command announced 24 aircraft downed, 16 in aerial combat and 8 by flak.

Thursday, May 30th, 1940 - Day 5

Weather again favored the Allies as poor visibility once again limited the Luftwaffe in their attacks and calm seas aided the evacuations. Two Do 17s were shot down to no RAF losses. More small ships arrived and troops on the beaches constructed makeshift piers out of damaged equipment, speeding up the loading process.


In total, 53,823 troops were evacuated that day.

Friday, May 31st, 1940 - Day 6

Another 68,014 troops were evacuated. The French 1ère Armée surrenders at Lille, 35,000 troops are captured.


The scene on the beaches became hell as dead bodies were piling up, and the remaining troops had barely eaten anything for the past four days.

The sea was also very rough that day. Some troops who were happy to have been picked up by the smaller boats were thrown back in the water when their boats capsized. They had to wade or swim back to the shore. The smallest boats were unable to pull back fast enough against the tide and were left stranded on the beach.


From the west of Dunkirk, the German army intensified their artillery shelling. The Germans nearly broke through at Nieuwpoort.

Had it not been for the first example of Allied close air support that day, the Germans just might have succeeded in breaking through. Ten Royal Navy Fairey Albacores (obsolete biplanes that replaced the even older Swordfish) attacked German positions with 250lb bombs which forestalled their push. Later that day nine Skuas attacked German pontoon bridges acrosse the Nieuport Canal. They were jumped by Bf 109s, who managed to shoot down two and damage another. Hurricanes came to the rescue and downed three Messerschmitts against the loss of one of their own.

Army Cooperation Command dispatched Lysanders in an attempt to locate German batteries shelling the beaches, but lost three aircraft to Luftwaffe fighters.

When the Luftwaffe came up in strenght in the afternoon, RAF fighters only managed to destroy six bombers and four Bf 109s, against the los of six Spitfires, eight Hurricanes and five Defiants.

It was the RAF's highest loss of the campaign.

© British Museum

© British Museum - Troops wade through the cold North Sea waters towards one of the rescue ships

Operation Dynamo
Operation Dynamo
Makeshift pier constructed out of damaged vehicles and equipment by the Allied soldiers.
© British Museum
Operation Dynamo
Operation Dynamo
Map showing the small bit of real estate the Allied soldiers defended during the rescue
© History Dept. of US Military Academy
Operation Dynamo
Operation Dynamo
Black smoke rises from the fires of the French oil storages in Dunkirk harbor.
© British Museum
Operation Dynamo
Operation Dynamo
Allied troops awaiting to be rescued. In the background, there is not much left of Dunkirk.
© British Museum
Operation Dynamo
Operation Dynamo
French destroyer Bourrasque hit a mine in the area of Ostend, Belgium, on May 29th. It sank the next day.
© British Museum
Operation Dynamo
Operation Dynamo
British troops on board a destroyer are waiting at the Mole jetty to be taken to safety.
© Western Mail Archive
Operation Dynamo
Operation Dynamo
Some of the ships safely arriving in Dover, May 31st.
© British Museum
Operation Dynamo
Operation Dynamo
British troops crowd the deck of a Royal Navy destroyer at Dover, 31 May
© British Museum
Operation Dynamo
Operation Dynamo
During their retreat, the British had to leave behind all of their precious armor.
© British Museum
Operation Dynamo
Operation Dynamo
Commander of No. 257 Sqn RAF Robert Stanford Tuck in his Hurricane showing 23 air victories, of which 7 were won over Dunkirk. The remainder were achieved during the Battle of Britain.
© British Museum

Saturday, June 1st, 1940 - Day 7

Another difficult day for the Allied troops as the weather cleared again, allowing the Luftwaffe to increase its attacks once more. Nine Stukas attacked and sank the British destroyer HMS Basilisk and later destroyed a second, HMS Keith.

Later on the day, the destroyer Keith was bombed and sank, a troop transport, the Scotia, carrying about 300 French soldiers, and a mine sweeper were both destroyed. Nearly 1,000 men were killed.


The Luftwaffe destroyed 13 British warships in total. They rallied 160 bomber and 325 Stuka sorties, the largest number of the campaign. The RAF flew 267 fighter sorties that day over eight missions, but the gaps between sorties were too long to inflict significant damage to the Luftwaffe.

The Luftwaffe lost two bombers, two Stukas, seven Bf 109s and three BF 110Cs.

The RAF lost six Hurricanes and ten Spitfires that day.


For the men operating many of the vessels, the strain of continuous operations was approaching its breaking point.  For a week, they had been crossing the Channel under constant shelling, bombing and strafing, whilst German U-boats and torpedo boats were also on the prowl.

Another 64,429 troops were evacuated.

Sunday, June 2nd, 1940 - Day 8

The evacuation of the British Expeditionary Force (BEF) is complete. Another 26,256 troops were saved.

In bombing and artillery spotting raids, the British lost three bombers to German AAA. In several dogfights during the day, the British lost seven Spitfires and two Hurricanes for the loss of a single Bf 109E. The RAF did manage to down eleven bombers.

Monday, June 3rd, 1940 - Day 9

This day, 26,746 French troops were evacuated. The Luftwaffe launches Operation Paula, a maximum offensive against French fighters and troops around Paris. The RAF lost one Hurricane to Bf 109Es over Dunkirk.

Tuesday, June 4th, 1940 - Day 10

The last night of Operation Dynamo. At dawn, the destroyer Shikari, filled with French soldiers, made the last departure from Dunkirk. About 26,175 French troops were rescued.

Shortly thereafter, French General Maurice Beaufrère, whose 68th Infantry Division had resisted the German advance on the western perimeter, surrendered and Operation Dynamo was over. Approximately 40,000 French troops were captured.


Over the nine days of operations, the RAF carried out 171 reconnaissance, 651 bombing and 2,739 fighter sorties, losing 177 aircraft, including 106 fighters, bringing the total number of fighters lost in the whole Battle of France campaign to 250. The losses over Dunkirk reduced the strength of Fighter Command to 570 operation fighters; 280 Spitfires and 290 Hurricanes, the latter of which included three squadrons in France. The RAF also suffered a great deal of losses, albeit obsolescent Fairey Battles and Gloster Gladiators during the defense of Norway in April and early May.


The mission of the RAF was to destroy German aircraft attacking the troops on the ground. Whereas the RAF only used fighters in the aerial skirmishes, the Luftwaffe mainly used bombers. It was only when a high number of bombers were lost to the British fighters, that the Luftwaffe started throwing a large amount of fighters into the mix to protect their bombers. The Luftwaffe also lost around 240 aircraft during the fighting over Dunkirk.


Although the RAF received much criticism over their so-called absence over the beached of Dunkirk, they made an enormous effort and endured much sacrifice, to the point of almost endangering the air defenses of Britain itself.

The small boats played an important role in the evacuation, but did not account for the largest part of the evacuated soldiers as is often stated. Of the 338,226 troops, about 239,465 (over two-thirds) were safely evacuated via the mole and about 98,761 via the beaches to the north.

Dunkirk left the British military vulnerable.

The BEF had abandoned or destroyed almost all of its equipment and the RAF suffered heavy losses.


Had Germany immediately pressed on with attacking England, they most likely would have succeeded. Postponing Operation Sealion (as the planned invasion of England was dubbed) until early August provided Fighter Command with valuable time to replace many of its losses, train new pilots and augment aircraft production.


Now, most of the experienced Allied troops survived and were able to fight another day. It was one of those pivotal points in the war where England nearly lost.



- Norman Gelb - Dunkirk, The complete story of the First Step in the Defeat of Hitler

- Norman Franks - Air Battle for Dunkirk

- Douglas C. Dildy - Dunkirk 1940 - Operation Dynamo

- British Museum

- Personal notes from visits and tours to Dunkirk

Back to the NA-73X story ... During the month of May, engineers and staff at NAA were working double time on the new fighter project, pulling 7 days a week, often untill late at night. Only on Sundays would they stop working at 6pm to acknowledge a bit of weekend time.

The initial date of the contract number A-250 for the NA-73X prototype and 320 NA-73s was on May 23rd, although the definitive version was not signed untill May 29th. Just two days later, first metal was cut to start building the prototype.

The Air Corps was meanwhile looking into a way to increase the range of its escort fighters. When Kelsey returned from a trip overseas, he realized that the capabilities for ferrying or combat would have to be greater than what was possible at that time. In June, both Republic and Lockheed were asked to undertake a project to increase the ferrying range of both the P-47 and P-38 from 2,500 to 3,000 miles.

On June 10th, Italy joined the Axis powers and declared war on France and Britain. France officially surrendered on June 22nd.

After the fall of France, the British took over the French aircraft contracts in the US and increased its own orders to a total of about 14,000 aircraft. Taking over the French orders required extra work by the Burtonwood repair depot as those aircraft were built to French standards with cockpit instruments in metric units and throttle levers pulling rearwards to increase power.

The British now stood alone against Nazi Germany and feared an impending invasion attempt as Germany could now move their fighters and bombers to airfields in France, within a short distance of the British coast. Various facilities for seaborne invasion were also being built up on the Belgian coast. As mentioned in the story about Dunkirk above, the RAF were at this time also short on aircraft and pilots, and were not ready to deal with a massive German invasion at this point.

They caught a break as German High Command would postpone the invasion until August.

Schmued finished his last engineering drawings around August 1st, which were subsequently sent to NAA's Experimental Department. NAA requested a wooden mock-up of the Allison V-1710-39 engine when construction of the fuselage began in order to make sure that the engine would fit properly into the tight cowling. They also reminded Allison multiple times that a "live" engine would be required to meet the scheduled installation date of mid-August, approximately 120 days from project go-ahead. (29)

Another milestone for NAA followed on August 19th, when the B-25 prototype made its maiden flight.

On September 6th Kindelberger sent the following Memorandum to all Directors: "The Pursuit airplane is now ready to fly at any time we get an engine. The last promise for delivery here [from Allison] is September 21st so we will be fortunate to have the airplane flying by October 1st. Actually, we would have been able to fly the airplane on August 20th had Allison been able to deliver the engine to our expectation. The British Air Ministry visited our plant last week and expressed their surprise and approval at the way the airplane had developed in that time. We are now under negotiation for an additional 300 of these airplanes which, with spares, will reach about fifteen million dollars without engines and propellers." (30)

Also on September 6th, the USAAC issued a contract for a Pratt & Whitney R-2800 powered version of the XP-47, designated XP-47B.

The first of the many: NA-73X


Incredibly, on September 9th, 1940, just 102 days after their promise made to the British, the North American Aircraft company delivered. NA-73X was rolled out of the NAA factory at Mines Field in LA (remember, this was a tall order at the time for a company that had never build a fighter aircraft before), although incomplete in some details and sitting on tires and wheels from the Harvard production line. It had no armament installed and also had no engine…

Allison had not delivered the engine on time and it would be another 20 days before the V-12 arrived to be mated to the airframe. According to Schmued the reaction at Allison was: "nobody ever designed an airplane in 100 days!"

In the meantime, NA-73X was fitted with a mock-up engine and three-bladed propeller.

On the other side of the Atlantic, the Battle of Britain was at its height...

When Britain stood nearly alone, Hurricanes and Spitfires defeated the Luftwaffe’s air offensive. The British Prime Minister, Winston Churchil,l summed up the battle with the words, "Never in the field of human conflict was so much owed by so many to so few". On September 15th, the Luftwaffe embarked on their largest bombing attack yet, forcing the engagement of the entirety of the RAF in defence of London and the South East, which resulted in a decisive British victory that proved to mark a turning point in Britain's favour. That day would be officially known as "Battle of Britain Day".

By the end of September, the United States Army Air Corps prepared to expand its fighter force by ordering 610 more Lockheed P-38s, 623 more Bell P-39s, and 540 more Curtiss P-40s, all equipped with Allison engines. These were to be built in 1941 and 1942, side by side with the aircraft already sold to the British.

Republic, who had no RAF orders, got the largest Army fighter contract, for 820 air-cooled fighters of the yet unseen P-47 type. As has been the case up till that date, North American Aviation was again not on the Air Corps fighter production program.

NAA's fortune was changed on September 20th, when US Assistant Secretary of War, Robert P. Patterson of the US War Department, officially approved the BPC/NAA/USAAC contract for the manufacture of 322 NA-73 airplanes: 320 for Great Britain and two freebies for the USAAC.

Meanwhile the USAAC was still struggling to find a suitable fighter for escort duties. Plans Division issues FM 1-15 "Tactical and Technology of Air Fighting" where two central roles for pursuit were identified as "1. Deny the hostile force freedom of the air and 2. Provide bombardment escort into hostile skies". The fighter that was most suitable for escort duties according to the report was the P-38.

The USAAC top brass was however still convinced that their heavily armored bombers could take care of themselves, without the aid of any escort fighters.

It was also concluded that the current technology and current designs for single-engined fighters lacked the fuel capacity to accompany the bombers on long missions and to be able to engage with enemy fighters over the target. The usage of external fuel tanks was not allowed over enemy territory at that time.

According to NAA records, around 78,000 man-hours of engineering were put into the development of the NA-73X.

NAA was still awaiting a V-1710-39 engine for its prototype. The engine was rated at 1,100 hp, however, its critical altitude (this is the altitude at which the performance would start to diminish)  would be limited to 11,000 feet. This would prove to be a big setback for NAA at the early stages because it would cause a huge disadvantage to enemy fighters which held powerplants with a higher altitude rating.

Front and rear view of the Allison V-1710-39 engine

NAA and the BPC closed negotiations for a second production block of 300 NA-73s on September 24th. This was put into contract A-1493, with the aircraft scheduled for final delivery in July of 1942.

The following day, Kindelberger wrote the following Summary of Operations for Directors: "NA-73 – Great Britain (320 Allison single-engine pursuit) – Progress on this contract has been materially halted by failure of Allison Engineering to deliver engine required for our first airplane. Delivery is now promised for beginning of October. Engineering on this contract is approximately 15% complete, while total contract is approximately 1% complete. Delivery of first airplane is now scheduled for January 1941; however, it is hoped that this airplane will be ready for test flight considerably in advance of this date."

NAA hired freelance test pilot Vance Breese, a colorful and often outspoken personality, to do the first test flight. At the time, several freelance pilots vied for such test work (and there was a lot of it), and the pay was often very lucrative and much more than a pilot on the company payroll would make. NAA had previously used Breese to test fly Schmued’s NA-35 trainer.

On a side note, on September 28th, 1940, North American broke ground on the new plant in Dallas, Texas. It was constructed of concrete and steel and was the first windowless, fully air-conditioned and artificially lit aircraft production facility in the US.

Back at NAA, the NA-73X sat in Inglewood's Hangar No.1 until, on October 7th, a single Allison V-1710-39 (F3R) finally arrived, albeit a loaned example by the USAAC to help NAA recover from Allison’s failure to deliver the engine. It came from an order recently filled by Allison, and although the engine met the installation requirements originally passed to NAA by the manufacturer, the first V-1710-39/FSR for the X73 had changes that had not been communicated to NAA. As a result, the Experimental Shop was forced to make design changes to the engine bay cowling and engine mount clearances owing to interference from electrical bundling. The engine was installed within 24 hours and NA-73X taxied under its own power for the first time on October 11th.

The prototype was assigned Civil Aeronautics Authority (CAA) registration NX19998, and was declared “ready to fly” on the 26th.


It was the morning of October 26th, 1940. NA-73X was parked on the ramp as Breese, who wore his usual double-breasted suit, climbed into the cockpit to start the flight that would lift NAA to new horizons. After mentally going over the checklist he hit the starter. The Curtiss-built propeller jerked a few times and the Allison burst into full staccato life, with just a hint of smoke dissipating from the exhaust stacks. The powerplant had been warmed up by the mechanics earlier that morning.

Breese began to taxi the prototype, the long nose obscured forward vision on the ground, which made S-turning mandatory. Pointing the nose into the wind and standing on the brakes, Breese gave the Allison a thorough run-up. Once satisfied, the pilot took the active runway and moved the throttle smartly forward. After rolling 100 feet, he brought the engine up to full power and pointed the nose down the centerline of the runway, shoving in right rudder to counteract the rapidly increasing torque.


The lightly loaded NA-73X (weights were 6278 lbs empty and 7965 lbs normal loaded) was quickly airborne, but the first flight was a rather sedate affair, Breese keeping within gliding distance of Mines Field. After flying for about five minutes, he came back to land. He went over initial handling data with Schmued and others, and then Breese went off for his second flight, which was about 10 minutes in length. He found that the gleaming craft exceeded initial performance estimates. Throttling back, he brought the NA-73X in for a smooth landing.


Six more functional test flights took place (October 31st and November 4th, 8th, 11th, 12th and 13th). These were made by Breese to fulfill his contract obligations and he was then on to other testing projects after putting some 3 hours 20 minutes on the airframe.

On November 19th, Kindelberger sent the following Memorandum to Directors: "NA-73 – Great Britain (320 Allison engine pursuit plane) – the first ship on this contract is now being test-flown. This has been made possible by the fact that the Army has loaned us the Allison engine necessary to complete the construction of this airplane for flight testing. Engineering is now 45% complete. Delivery of the first airplane is scheduled for January 1941, with an accelerated delivery thereafter, shipments being scheduled for completion on this contract by September 1941, provided engine deliveries are sufficient to keep pace with this schedule."

However, Breese had numerous complaints about the aircraft and it required time to correct them. Breese, an excellent self-promoter, was pretty much a very good seat-of-the-pants pilot. What the NA-73X needed, however, was an engineering test pilot.


NAA was fortunate in having access to an excellent wind tunnel facility at the California Institute of Technology where extensive tests were undertaken to prove various NAA concepts.

Vance Breese in NA-73X after first 5 min test flight Oct 26th 1940

Back at Mines Field, Paul Balfour (commercial pilot certificate number 12596) was hired by the company in 1936 and eventually assigned the post of chief test pilot for the NA-73X. Before his death in the early 1970s, Breese stated that he had bet money with NAA officials that Balfour would crash the aircraft on his first flight. Breese won.


On the morning of November 20th, 1940, the ground crew prepared the NA-73X for Balfour’s flight. Schmued would later recall, Before this flight, I asked Balfour to get into the airplane and go through the routine of a takeoff and flight. He responded that one airplane is like the other and he would not need the routine check out.”


Balfour’s first flight was also scheduled to be a high-speed test run for the NA-73X. The pilot took off at approximately 0710. Mechanic Olaf T. Anderson later stated that the engine had run fine on the ground and, “at about 0540, I warmed the engine up as is the usual procedure before the flight. Oil and Prestone temperatures were normal (oil 65 degrees C; Prestone 95 degrees C). Oil pressure was 80 pounds and fuel, 13 pounds. The engine was mn for five minutes and then shut down. When I started the engine for Mr.Balfour before takeoff, it was a little hard to start (the Allison representative said their engines have a tendency to do such).


As Balfour pulled the gleaming fighter up after about 12 minutes flying time, the Allison suddenly stopped running. Checking the instruments, nothing seemed amiss. However, executing a wide sweeping turn caused the NA-73X to lose altitude and Balfour quickly realized he was not going to make the runway. During the last portion of the turn, he dumped landing gear and flaps as he directed the stricken prototype toward a plowed field just west of Lincoln Boulevard. The now-glider whistled down in a correct landing attitude, but as soon as the tires touched the soft plowed soil (at approximately 0723), the NA-73X violently flipped over. The built-up structure behind the pilot saved Balfour from being crushed, and the pilot scrambled for safety' from the movable side window.

The reason of the crash is a much debated one.

The official report stated that Balfour did not change the fuel selector switch and this the engine starved by the lack of fuel. But why did a seasoned test pilot, even though sources claim he took a casual approach to the test flight, make such a rookie mistake?

When NA-73X was rebuilt, the air-inlet-scoop was moved as far forward as possible. It was noted that, at high angles of attack the airflow was cut-off, thus starving the engine of air.

Some state that there is a possibility that NAA did not want to risk the project because of the design and “blamed” Balfour for the crash?

There is a report of an eye witness to the crash (an NAA employee) who stated

that the crash happened shortly after take-off and that, upon examination of the fuel tanks, fuel was in the auxiliary and right tank, but there was no fuel left in the left tank. If that is true and the crash happened shortly after take-off, the aircraft was given to Balfour with little fuel in the left tank. That fuel would have been used up during taxi, run-up and take-off (Either Balfour wasn't told correctly about the fuel status, he selected the wrong tank or ground personnel did not fill as instructed)

Balfour was killed on November 10th, 1951 while test flying a modified CB-25J for NAA.

At the time of the accident, the prototype had accumulated just 3 hours 20 minutes of flying time (Balfour had logged 2,298.40 hours of solo tune at this point). The Civil Aeronautics Authority Air Safety Board listed damage to the aircraft as “engine housing broken, both wingtips damaged, tail surfaces damaged, top of fuselage damaged, and other miscellaneous damage.”


Investigation of the crash revealed that the Allison had run dry when the selected fuel tank had been allowed to be completely depleted. NAA and the British both agreed that, in spite of the crash, they had a winning aircraft and the accident was no way the fault of the design.


Some aviation historians have recorded that the prototype was scrapped after the accident, but this was not the case. Actually, the prototype was carefully raised out of the bean field by crane and transported back to NAA where it was stripped apart and rebuilt in a very short time. However, to increase the pace of flight testing, the first RAF machine was completed and joined in the flight program. NA-73X would fly again on January 13th, 1940, with Breese at the controls.

Since the NA-73X encountered very few problems during following tests, production for the RAF begun almost immediately. The first production aircraft would be used for test flying, rather than awaiting repairs on NA-73X. Flight testing had to be done in a relatively short time because the schedule of finishing the first production airplane in one year was pressing hard.


Meanwhile, following the arrival of this official British Purchasing Commission communique at NAA on December 12th, 1940, the NA-73 officially became known as the Mustang:

The second batch of 300 aircraft ordered by the British would still be called Mustang I, but would be built by North American under Charge Number NA-83 as these 300 aircraft had been ordered under a new contract.  They differed from the 320 NA-73s only in minor details, including the fitting of more efficient exhaust stacks (flared from top to bottom versus the more streamlined initial version), and incorporated manufacturing experience gained during the course of production of the first batch, which included the modified carburetor air scoop.

NA-73X back in the shop for landing gear drop testing and a new engine

On January 4th, 1941, the CAA reissued the Certificate of License for the NA-73X. The first following test flight would be planned on the 13th. That day, Vance Breese flight tested the newly repaired NA-73X out of Mines Field.

This was its first flight since it was crash-landed by Paul Balfour nearly two months earlier. Breese took over until NA-73X was returned to the shop for drop tests on the landing gear and a new production engine.

In the ever ongoing quest for range, on January 7th, R. C. Costello, Resident Technical Officer for the British Air Commission (BAC), formed in October 1940 in Washington D.C., sent a letter requesting that NAA prepare a report on the maximum possible range of the Mustang at different speeds and maximum possible take-off gross weight, with all equipment removed save radio and navigational equipment.

On January 13th, the BPC approached NAA with another request to augment the range of the Mustang: minimum armament would be modified to two 0.50cal cowling-mounted guns, with extra fuel to be carried internally in the gun bays after ruling out lengthy design changes required for the wings, namely the lead times to design and test a bomb rack in the wings for external fuel tanks, plumbing the wings for fuel lines, and re-stressing the wings.

Another request was that the windshield and armor provisions be changed.

US-based suppliers for RAF-mandated government-furnished equipment (GFE) were also secured. These included Curtiss propellers, radio equipment, fluorescent lighting, and Browning machine guns.

Schmued also advanced the notion of external fuel tanks and racks when asked by the USAAC-MD to comment on the CP39-770 fighter competition. However at that time, and until 1942, USAAC regulation forbade aircraft to be equipped with external tanks for combat. 

The proposed fuel system would comprise out of two small fuel cells for the ammunition compartment and a larger one in the gun bay. Each rubber tank would connect with the main tank and feed via gravity. This would bring the estimated range to 1,724 miles with 220 gal of fuel. In the end, no report exists that the modifications were ever made.

The final performance and gross weight specifications were also set. Except for minor changes, NA-1620 remained unchanged with respect to top speed guarantees, weight and balance, and range with both the “normal” 105gal of fuel as well as with 157gal. All agreed changes were incorporated into the 1620 Specification as revisions.

On January 18th, Rice noted in a memo to Engineering that the GALCIT tests concerning heat rejection data for the X73 cooling system pointed to the need to expand the intake scoop area to 150 inches, dropping the inlet air intake from the bottom surface of the wing and giving the variable aft plenum gate/exit a range of 65 to 280sq in. Britain’s Air Ministry was contacted for support to assist in the redesign of the scoop.

On January 20th, 1941, Kindelberger wrote to Breech: "We have a check on the pursuit plane for the last couple of days which indicates it will be much faster than our guarantee and there is a possibility that with future improvements it will be a real 400-mile-an-hour pursuit plane, which is something that does not exist at this moment in this country with anything like the armament and armor, etc. … The first pursuit planes will be a little late, but will be in step with the engine deliveries."

That same day, Rice wrote to R.C. Costello that NAA findings pointed to a requirement of 200US Gallons for 1,500 miles, including warm-up, take-off and climb to 10,000ft at 75 percent rated power. The gross weight was estimated at 8,400lb, with a take-off run of 1,600ft to clear a 50ft obstacle.

Quotes from the Firestone Tire and Rubber Company were requested for leak-proof fuel cells, including a 200hr test that would be carried out with 100-octane fuel.

John Young of the NA-73 powerplant department issued a memo describing the fuel system as comprising two small cells for the ammunition compartment and one large on for the gun bay. The estimated range for the Mustang I with the auxiliary fuel cells was 1,724 miles with 220gal, for an equipment cost to NAA of 125$.

On January 21st, XX73 (the incomplete duplicate airframe of NA-73X primarily used for weight and load testing) experienced some damage to its structural integrity at both its design ultimate angle-of-attack loading and its peak side load, which caused buckling of the spar and the airframe. The production airframe project engineers, working with structures, beefed up the longerons, the spar, and some skins. The design changes were incorporated starting with NA-73 production number 11.

Because of these findings, the X73 and AG345 to AG352, including the 4th & 10th airframe to be delivered to the USAAC (41-038 and 41-039) were deemed “disqualified” from a combat role and relegated to flight-test status.

Following repair of XX73, it was turned over to NAA Field Services to produce maintenance and repair instruction manuals.

When NA-73X returned to flight status, Air Corps Captain M. J. Lee made seven flights beginning March 16th, but no record of his impressions has been found.

Robert C. “Bob” Chilton was hired as chief test pilot to replace the unfortunate Balfour, and a study of Bob’s logbooks indicates he flew the rebuilt NA-73X on April 3rd, 1941 for a 1-hour familiarization flight from Mines Field. Chilton also recalled that the NA-73X had made between five and six flights with another pilot immediately after its rebuild. Chilton went on to make at least a dozen more flights with the aircraft.


Bob later remarked, “The NA-73X was a clean-flying aircraft with no bad vices. It was quite pleasant in the air and handled very similar to the later production articles.”

Chilton had accrued considerable fighter experience in the Air Corps before going to NAA, flying the Boeing P-12 and P-26, the Curtiss P-36, and other fighter types. His expertise in the fighter field enabled the engineers to incorporate changes that would be beneficial to the combat pilot.


“I recall that the NA-73X was just pushed to the side after it had been retired from its last flight,” stated Chilton. “It probably ended up on the company’s junk pile, but I do not recall seeing it there. The NA-73X was a very attractive aircraft and its aluminum skin glowed with constant waxing by George Mountain Bear, an American Indian whose duty was to keep the airframe as clean as possible to pick up those few vital miles per hour.”


With the first RAF aircraft coming off the production line, NAA and the British decided to use these airframes for continued testing. “The ‘old’ NA-73X was no longer representative of the design,” said Chilton. “We had orders on our hands for hundreds of new fighters and the NA-73X had served its purpose. It had established the trend for what I believe was the finest propeller-driven fighter ever built by any country.”

The first production line Mustang, given British serial number AG345, was completed mid-April and took its first test flight on April 23rd, 1940, at the hands of Louis Waite.

The NA-73X continued to operate as part of the NAA’s development program until July 15th, 1941, when it was grounded indefinitely.

It was the only one of its kind and became the front runner of one of the most successful fighters in World War II.

Research indicates that the NA-73X, stripped of useable components, may have been donated to a local trade school.

As for the rest of Mustang development history, the next chronological order is be the production of the Mustang I for the RAF, as well as the testing of both XP-51 airframes by Materiel Command.

Serial Numbers


US Serial Number



Credits & Bibliography

This article came to be because, in not one single book or on a single website, we could find a complete, detailed documentation on the history of the Mustang and in this case, of the NA-73X.

The text in the NA-73X history above comes from various sources and was cross-checked for corectness.

Major sources for this article were the following books:

  • "P‑51B Mustang: North American’s Bastard Stepchild that Saved the Eighth Air Force" by James William "Bill" Marshall, Lowell F. Ford, Col (Ret.)

  • "Mustang: Thoroughbred Stallion of the Air" by Steve Pace

  • "Mustang Designer - Edgar Schmued and the P-51" by Ray Wagner

  • "High-Spirited Mustangs - Vol 1" by René J. Fancillon

  • "Production line to frontline - P-51 Mustang" by Michael O'Leary

(1)  Maurer, Maurer - Aviation in the U.S. Army, 1919–1939, Office of Air Force History, Washington, D.C, p. 196

(2)  Army Air Forces Statistical Digest, World War II - Office of Statistical Control, Headquarters AAF, Washington, D.C. December 1945

(3) James William Marshall - P-51B North American's Bastard Stepchild that saved the 8th AF, p.32

(4) James William Marshall - P-51B North American's Bastard Stepchild that saved the 8th AF, p.33

(5) Mark Lorell and Hugh P. Levaux - The Cutting Edge: A Half Century of Fighter Aircraft R&D

(6) Peter C. Smith - North American T-6, p. 23

(7) K J Meekoms and E B Morgan - The British Aircraft Specifications File : British Military and Commericial Aircraft Specifications 1920-1949 (the leading letter was usually present to identify the aircraft purpose, B for "heavy bomber", ,for "medium bomber", F for "fighter" and A for "army co-operation" - the first digit was a number identifying it in sequence and then after the slash, the year it was formulated)

(8) James William Marshall - P-51B North American's Bastard Stepchild that saved the 8th AF, p.53

(9) Matthew Willis - Mustang, the untold story, p. 15 & Lee Atwood - Origin and Evolution of the Mustang

(10) Jeffrey L. Ethell - Mustang, A documentary history, p. 10

(11) James William Marshall - P-51B North American's Bastard Stepchild that saved the 8th AF, p.63

(12) Wagner, Ray - Mustang Designer: Edgar Schmued and the P-51

(13) James William Marshall - P-51B North American's Bastard Stepchild that saved the 8th AF, p.64

(14) John M. Haight - American Aid to France (1938-1940)

(15) AAF Reference History No. 6, Distribution of Air Materiel to the Allies 1939-44: Controls, Procedures, and Policies

(16) Lee Atwood - Origin and Evolution of the Mustang

(17) Peter M. Bowers - Curtiss Aircraft 1907-1947; Paul A. Ludwig - P-51 Mustang, Development of a long-range escort fighter; Wagner, Ray - Mustang Designer: Edgar Schmued and the P-51; Joseph R. Chambers - NACA, Cave of the Winds

(18) Wagner, Ray - Mustang Designer: Edgar Schmued and the P-51

(19) NACA - On the Wings, Vol. II

(20) NA ??

(21) NACA - On the Wings, Vol. II

(22) Wagner, Ray - Mustang Designer: Edgar Schmued and the P-51

(23) WLowe, Malcolm V. - North American P-51 Mustang; Wagner, Ray - Mustang Designer: Edgar Schmued and the P-51; Ed Rees -  “A Tribute to Dutch Kindelberger: The Mustang—A Great War Horse"

(24) Robert W. Gruenhagen - Mustang;: The story of the P-51 fighter

(25) Wagner, Ray - Mustang Designer: Edgar Schmued and the P-51

(26) Wagner, Ray - Mustang Designer: Edgar Schmued and the P-51

(27) Lee Atwood - Origin and Evolution of the Mustang

(28) Robert W. Gruenhagen - Mustang;: The story of the P-51 fighter

(29) James William Marshall - P-51B North American's Bastard Stepchild that saved the 8th AF, p.96

(30) James William Marshall - P-51B North American's Bastard Stepchild that saved the 8th AF, p.96