NA-73X
Crazy Horse Aviation Photography

NA-73X

How it all began


By the end of 1937, the increasing threat of German rearmament was alarming both the French and British governments. As a response, they started to boost their own production of aircraft. Since domestic production capacity and available means were proving to be insufficient, they also sought aid overseas, with American sources.

 

In April of 1938, the British Air Ministry dispatched a British Air Commission to the USA led by Sir Henry Self, with the sole task to explore the possibilities of purchasing military aircraft  across the Atlantic.

 

Even though, at that time, the USA was adopting a strict policy of neutrality, there were no rules to prevent friendly foreign countries from buying American military products. American aircraft manufacturers were leaders in several techniques of metal airframe construction and those designs that had been seen in Europe had received a great deal of praise.


The commission made a tour throughout North America with visits to every airframe and engine manufacturer of significance, all of whom were very keen to do business. All of the companies visited by the British commission presented themselves well but it was both the Lockheed Aircraft Corporation and North American Aviation, both located in California, that were selected.

 

Lockheed had already won an order for 200 light bomber and reconnaissance Hudsons and NAA for

400 NA-16 tandem-seat trainers, for which the RAF adopted the name Harvard Mk I.

North American Aviation


NAA was founded on December 6th, 1928, by Clement Melville Keys, as a holding company for several different aviation companies. However, the Air Mail Act of 1934 forced the breakup of such holding companies. A year earlier the General Motors Corporation took a controlling interest in North American Aviation and merged it with the General Aviation Manufacturing Corporation, but the company retained the name North American Aviation or NAA in short.

 

Thus NAA became a manufacturing company, run by James H. "Dutch" Kindelberger, who had been recruited from Douglas Aircraft Company.


NAA built a brand new factory at Inglewood, near Los Angeles, and personnel were selected from manufacturers within the old group. The move of the company's operations 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 (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.

On January 15th, 1935, the USAAC-MD released two open Requests for Proposals (RFP):


  • Circular Proposal 35-414 for X-602 specified a two-seat, single-engined pursuit airplane with bomb-carrying capability
  • X-603 was a request for a single-seat, single-engined pursuit airplane. It demanded a profile layout and performance specifications for an all-metal-covered cantilever monoplane with retractable main gear.


With the Allison V-1710 powerplant in mind, NAA decided to submit a proposel for a dual-seat, in-line engine pursuit.


NAA noted the emergence of Allison’s powerful V-1710, and this led to the company submitting a proposal for the two-seat, in-line-engine pursuit.  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 prefered the Seversky P-35 over the Curtiss P-36, both aircraft being conventionally powered by radial engines.


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.


On January 25th, 1935, NAA issued project NA-16 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.


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. In March of 1936, the first Spitfire prototype flew from Eastleigh Airport in New Hamshire. Also, in the fall of 1936, Kindelberger met and established a relationship with WWI ace Oberst Ernst Udet at the Bendix Trophy Races at Mines Field. When Kindelberger went to Europe in June of 1938 at the behest of Brig. Gen. Hap Arnold, Udet personally hosted him when he inspected the Heinkel, Messerschmitt and Junkers plants, giving him a first-hand view of German manufacturing techniques. Udet mentioned to Kindelberger that assembly line techniques from the American automobile industry had been introduced into German aircraft production. The trip convinced him that Messerschmitt's construction processes were superior in respect to simplicity, as well as capacity to rapidly produce aircraft.

While Kindelberger was in Britain, he closed a deal for the Harvard I.


NAA decided to focus on training aircraft, as there were other manufacturers 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.


Also in 1936, both the Curtiss Model 75 (aka P-36 Hawk) and the Seversky P-35 were aqcuired for testing by the USAAC. In 1937, the second Curtiss Model 75 (named Y1P-36) was modified for the new Allison V-1710-11, with an exhaust driven turbo-supercharger and was given the designation XP-37.


In February of 1937, Lieutenant Benjamin S. Kelsey, Project Officer for Fighters at the United States Army Air Corps (USAAC), 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).


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


Lockheed 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 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.


In early 1938, 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, requesting a study for an optimized "High Speed Pursuit" based on a 1,050hp in-line engine.

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


Several other key dates in 1938:


  • In June, Edward Horkey joined NAA as aerodynamicist, by recommendation of Millikan.

  • On September 28th, Louis Sandford Wait took the Harvard I for its maiden flight. Two weeks later, design data and performance estimates for model NA-53 were also completed.

  • 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.


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, the 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.


When Kindelberger returned after his European trip, 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.


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.


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


The British were looking for new fighter aircraft. The British Purchasing Commission was an organisation which was tasked with making special purchases of aircraft in the USA, whilst also exploring the possibilities of manufacturing British aircraft types under licence.


On January 5th, 1939, they delivered their “High Speed Single Seat Fighter Specification, F. 18/29. Operational Requirement OR.73 for the Replacement of Spitfire and Hurricane” to NAA. 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.


Performance:
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.


Strength:
The design requirements of AP.970, any corrigenda 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.”


Armament:
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.


In March of 1939, the Army Air Corps issued a specification for a medium bomber. 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 become their successful B-25 design.


In September 1939, the Air Corps ordered the NA-62 into production as the B-25, along with the other new Air Corps medium bomber, the Martin B-26 Marauder "off the drawing board".


NAA's succes with their Harvard trainer also taught the company how to mass-produce aircraft.


It also helped establish NAA as a major aircraft manufacturer in the US. Britain  had  become interested  in  the NA-16, which  had turned  into  the AT-6  Texan, and  in  1938  began  ordering  the sturdy advanced trainer for the Royal Air Force (RAF) and other Commonwealth air forces as the Harvard. Within four months of placing the order, the first Harvard had arrived on British soil.

NAA BC-1
NAA BC-1
NAA's first attempt at a pursuit aircraft
Production of BT-9's by North American Aviation Inc., Inglewood, Calif
Production of BT-9's by North American Aviation Inc., Inglewood, Calif
AT-6 Texans fresh off the assembly line in Dallas - closest aircraft i
AT-6 Texans fresh off the assembly line in Dallas - closest aircraft i
MInes Field
MInes Field
NAA BT-9
NAA BT-9
NAA O-47A
NAA O-47A
North American NA-40 prototype NX14221
North American NA-40 prototype NX14221
North American NA-40 prototype NX14221
NAA NA-62 prototype 40-2165
NAA NA-62 prototype 40-2165
NAA NA-62 prototype 40-2165
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")
P-509
P-509
The P-509 “High-Speed Pursuit (Allison)” shown with imbedded radiator and oil cooler – the first attempt by NAA to incorporate a Meredith Effect cooling system into an aircraft
P-509 Mockup
P-509 Mockup
The mockup of the P-509/NA-50B which was built as part of the specification discussions that led to the creation of the X-73
NA-73X development
NA-73X development
These NAA concept drawings show the various stages before arriving with the X-73
P-509 drawing
P-509 drawing
P-509-1 "High Speed Pursuit" proposal
© Jim Laurier - Osprey Publishing

Other American aircraft, such as the P-38, P-39, and P-40, did not have those features, but the RAF deemed them essential for combat worthiness.


"When Dutch came back about two weeks later, we had no contract, but we started then on our own mock-up of the airplane", Schmued recalled. In three days we had it built, using paper, plaster of paris, whatever was suitable. Meanwhile, the British apparently looked at this proposal carefully and concluded that on the strength of North American’s past performance with the Harvard trainer, which was delivered in large quantities, we could be trusted to come through with a good airplane.


NAA issued Report No. 1592, which showed important design changed as opposed to their preliminary design data which was delivered to the BPC by Kindelberger in mid-March 1939:


  1. The gross weight was increased from 6,455lb to 6,540lb due to the addition of the self-sealing fuel tanks and armor plating, in accordance with known RAF requirements, as well as an increase in main-wheel tire diameters from 27in. to 30in.
  2. A significant performance increase due to the use of the newer Allison V-1710-39/F3. Take-off horsepower augmented from 1,000hp to 1,200hp, 1,050hp at a military rating for 3,000rpm at 16,000ft; 950hp to 1,050hp at normal power rating, with 2,600rpm at 12,500ft. All figures based on 90-octane gasoline.
  3. The wing area was changed to 180sq ft, with a 33ft span. Rate of climb and endurance decreased slightly, however, owing to higher wing loading and the slightly lower fuel capacity of self-sealing tanks.
  4. The two .50cal AN/M2 Browning machine guns were moved to the wing root, instead of under the cowling. They were still designed to fire through the propeller arc.
  5. A specification that the airfoil would be a “NACA series airfoil developed for high speed performance,” whereas the original 509-1 specified a NACA 2516-34 airfoil at the root and 2508-54R airfoil at the tip. The currently contemplated airfoil sections were NACA 23016 for the root and 23008 for the tip.
  6. The incorporation of Air Ministry Specification O.R.73, as applicable to further define P-509.


A second report, No. 1593 “High Speed of Allison-Powered, Export Pursuit,” one week later included the NACA laminar flow airfoil data by Edward Horkey and his team.


Accompanied by the report, data and drawings, Kindelberger and Atwood made their trip to New York City to visit the BPC on around March 20th. The British technical team, led by H. C. B. Thomas, exchanged queries and questions with NAA engineering regarding the approach to powerplant coolant and oil cooling systems.


The design proposal was developed to incorporate the benefits Meredith's “negative drag/positive thrust” research, such as the lower scoop, radiator/oil cooler, exit plenum, and variable-dimension rear scoop. The engine exhaust stacks required further investigation to achieve optimum effect (exhaust jet angle to airflow, number of exhaust stack exits, diameter of each exhaust) with minimum performance loss. Additionaly, the entire skin of the airframe would be flush riveted with butted joints that were then puttied, accentuating the clean mold lines and reducing both friction and profile drag. Lastly, all major air leaks (flaps, ailerons, elevator, landing gear, etc.) would be sealed off to further reduce parasite drag.


Finally, all of NAA's hard work prevailed.


At last the AFPB board was convinced that NAA might produce a fighter that was superior to anything else the US could offer. It certainly helped that NAA incorporated the British theory on the potential negative drag of the cooling system (the Meredith Effect). NAA's excellent customer relations and product reliability, all proved by the Harvard I production, provided the AFPB with enough confidence to give its support and recommend that the contract be awarded.


The approval to move forward was issued by Sir Henry Self at the New York City offices of the BPC, and Shop Order SC-1050 came through for construction of a mock-up of the P-509. It also authorized GALCIT wind tunnel tests to study drag for different armament configurations for the P-509. The model group were given orders to complete their mock-up by April 17th, 1940.


Also during the late March meetings between NAA and the BPC, Lt. Col. Echols repeated his request to NAA to build P-40s. The aim for this was that NAA could, in doing so, offload some P-40 production obligations, so that Curtiss could prepare for anticipated P-46 deliveries. He also kept in mind to obtain a Foreign Release Agreement waiver to export the P-46. The additional money could then be used to help pay Curtiss for adding armor and self-sealing fuel tank upgrades required by the RAF for the aircraft.


Kindelberger described how NAA would design a single-seat fighter from the ground up, using the same Allison engine fitted in the Curtiss fighter. Furthermore, they offered to design and deliver the first prototype within 120 days (which was equal to the time otherwise needed to prepare and convert the NAA plant for the construction of P-40's)!


The most significant difference between the two fighters would be that the NAA machine would have superior performance because of an extremely low-drag airframe. Learning from their Harvard I production process, NAA would also design their new fighter keeping mass production techniques in mind. The P-40 was not designed to enable an easy mass production. “The time to start worrying about pro­duction is before you begin product engineering,” Dutch said. The plane was designed from the early start to be made in many parts, with large units broken down into smaller components for ease of assembly and installation. The wings alone were made in six sections. Dutch brought Detroit techniques to Southern California.


By April 10th (the day after the invasion of Norway), the Anglo-French Purchasing Commission was told they could complete contracts for 2,440 American fighters. Bell Aircraft won the first new Foreign Release Agreement for a fighter on April 4th, 1940. That company had been facing a cash-flow crisis because the Army then paid only after the planes it ordered were delivered. Knowing that the Allies paid a sizable advance, Larry Bell hurried to the commission’s New York offices at 15 Broad Street.

Bell was offering the Model 14, an export P-39. Sales brochures promised that this Airacobra would do 400 mph at 15,000 feet with an 1,150-hp Allison. His offer was accepted and 170 Model 14s were ordered in May for the French, adding a lot of weight in armor, self-sealing fuel tanks, and four wing guns. These changes were also to be incorporated in the Army’s P-39D (Bell Model 15), to be built concurrently in 1941. And the French also decided to replace the 37-mm cannon with the 20-mm Hispano gun that was being ordered into production in America. In exchange for the Army’s permission to sell the Airacobra, Bell would design its radical Model 16 with a Continental pusher engine, which later became the Air Corps’ XP-52 project.

John Leland Atwood

Lee Atwood made several trips to the BDPC offices and was heavily involved in the negotiations, as described by the man himself:


“I made several trips to New York from January to April [1940] 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,

NAA President 'Dutch' Kindleberger & Vice-President John Leland Atwood

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”.

That “similar airplane” proved to be the Curtiss XP-46. It was this data that has caused a great deal of unnecessary conjecture as to the design origins of the Mustang. NAA never made a secret of the purchased material and it has also transpired that the USAAC insisted that the company should have the data at its disposal as well.


However it is very unlikely that any information gleaned from the XP-46 as a whole would have been of any practical assistance. The NA-73X was destined to leave the NAA as a considerably more advanced aircraft that the Curtiss machine. The Curtiss XP-46 was a scaled-down version of the P-40, incorporating many unique features such as an inward-retracting undercarriage, slotted wings, self-sealing fuel tanks, armor protection for the pilot and a radiator set below the fuselage directly under the cockpit. It was the latter feature that, externally, was the only similarity between the Curtiss design and the new NAA machine.

Curtiss XP-46

Chief aerodynamicist Ed  Horkey  examined the data and  found  it virtually  useless. Schmued  would  later claim he never even saw the Curtiss data.


The British were eventually sold to the idea and Wilfrid Freeman, head of the British Ministry of Aircraft Production, ordered 320 aircraft in March of 1940.


Upon Kindelberger’s return to Inglewood, he held meetings beginning on April 5th, 1940 with vice president Lee Atwood, chief engineer Ray Rice and chief of aircraft design Edgar Schmued. The main players at NAA involved in the NA-73X programme 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.


On April 10th, 1940, the proposal was accepted and work on the prototype (assigned North American's model number NA-73X on April 24th – 73 for the next available NAA charge model number and X for experimental) begun. The contract specified initial prototype delivery in January of 1941 and completion of 300 airframes by September 30th, 1941.


Note that there was never a mandate to complete and fly a prototype within 120 days!

 

The ball started rolling almost instantly after Kindelberger sent a telegram back to his designers at Inglewood on April 24th, 1940. 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.


Kindelberger caught a flight back to California, leaving Atwood behind in New York to continue negotiations. At Inglewood, the design teams continued working through the night on general arrangement drawings and a preliminary weight study, all in time for Kindelberger to view the results by 10 am on April 25th.


This resulted in 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).

NA-73X drawing

© NAA - Conceptual drawing of NA-73 as it appeared in the 24 April 1940 report No. 1620

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.


In the end, the armament package was changed and set to two 0.50cal guns in the engine compartment, plus an additional set of two 0.30cal and one 0.50cal in each wing. Main reason to abandon the AN/M2 Hispano 20mm cannon options is that there was simply a lack of availability. The cannons would only become available for future NAA projects until late 1941.

On March 11th, 1939, another three Requests for Proposals were issued by the USAAC-MD:


  • 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 permitetd to be designed around in-line engines. This made NAA go back to the before mentioned Millikan report specifying a 1,050hp in-line engine.


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.


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. 


In July, the British Purchasing Committee (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 not call for an actual prototype to be built, the design alone could 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.


In conclusion, by the end of 1939, US aircraft industry still failed to produce any single-engine airframes that matched the performance of the German Bf 109 or the British Spitfire. They also had not yet flown a radial-engined fighter to compare with the latest German entry, the FW 190, whose prototype took to the skies in June earlier that year. Also unknon to the USAAC or US Navy at that time was the presence of the Japanese Mitsubishi A6M Zero which first flew on April 1st.


In December of 1939, NAA came forward with the SC-46/P-500 Pursuit Trainer design.


In 1940, like other manufacturers, NAA would start gearing up for war, opening factories in Columbus, Ohio, Dallas, Texas, and Kansas City, Kansas.


Ernest Breech, the GMC board member, urged NAA to continue to focus on fighter aviation. On the other side, there was Gen. Echols, who tried to push NAA towards the development of trainer, observation and medium bomber aircraft.




A bold proposal


On the European continent however, nothing seemed to be able to slow down the uprising of the German military as one country after another fell under Nazi control. It would only be a matter of time before the British would be facing the German war machine alone.


When the invasion of Poland occurred on September 1st, 1939, both Britain and France honored their prewar agreements to declare war on Germany. An arms embargo by the USA was brought into force immediately but the President, Franklin D. Roosevelt, along with a large proportion of the Senate and Congress, was not comfortable with the original draconian measure. 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.


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.


The British began to realize its own need for American fighters in large quantity. 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 British needed them very fast and nothing was given higher priority than the acquirement of fighter aircraft. 


The only aircraft the US had at that time that suited this purpose were the Bell P-39 Airacobra and Curtiss P-40 Warhawk.  Curtiss was already heavily committed building the P-40 for the USAAC (United States Army Air Corps) and thus had no production capacity to spare.


In February of 1940, the French and British purchasing commissions (AFPB - Anglo-French Purchasing Board) revisited several American aircraft companies. They were is desperate need to step up fighter production capacity as the war in Europe continued to rage on. Kindelberger, Atwood and Rice 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.


Edgar Schmued and his team worked on several drawings for the 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.


When Kindelberger and Atwood traveled to New York City to meet with the 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 mix it up with the Luftwaffe frontline fighters.


When the meetings came to an end, the AFPB formally requested that NAA build P-40s instead.


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.


On February 28th, 1940, Sir Henry Self was appointed Director General of the BPC arm in New York City.


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 had no armor, no selfsealing tanks, and lacked sufficient firepower. Letting the Allies buy into the fighter program was a way of funding these crucial modernizations.


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


Britain was still looking to order the Curtiss P-40 and the Bell P-39 Airacobra. The Curtiss P-40 stood out as the best, as it was maneuverable and had a top speed of 340mph between 12,000ft  and 15,000ft. However, the staff of the BDPC was well aware of the limitations of the Allison engine, which without a supercharger was a poor performer above 15,000ft. Both aircraft, however, were already obsolete designs and were certainly no match for the Luftwaffe's Bf-109 Messerschmitt.


Since Curtiss was still unable to meet the demand, the British delegation, led by Sir Henry Self, who had a good working relationship with Kindelberger in terms of Harvard production, paid another visit to Kindelberger and again requested that NAA build P-40's under license from Curtiss for the RAF.


Their request was supported by Lt. Col. Oliver Echols of USAAC-Materiel Command.


Rather than reject the notion out of hand, NAA would responded with a bold and remarkable proposal...


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."


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.


Meanwhile, he had prepared the three-view drawings and reports of the new fighter. Dutch set on about his way to England, joined by Lee Atwood. 


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.


Between March 6th and 11th, further design changes were made in the wing area, self-sealing tanks and armor plating were added, and the new and more powerful Allison V-1710-39/F3R was incorporated.


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

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:


Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sept

Airplanes

1

5

20

40

50

50

53

51

50

Spares (*)

1

3

5

5

5

5

10

15

15

(*) 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.”

NAA mailed the British delegation drawings of a design concept on May 3rd, 1940, and on the 29th of May the British awarded contract number A-250 to NA for development of the NA-73X and the intended manufacture of 320 NA-73 airplanes.


Meanwhile on May 12th, Germany invaded France, which resulted in communications between NAA and the French coming to a halt.

 

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.


NAA's letter of intent stipulated the NA-50B, originally intented for export to South America. The NA-50B featured a radial engine. When Gen. Echols received the request for export release, he saw that the engine was different from what was stipulated. After consulting with his superiors NAA eventually received the go-ahead for exporting to the British.


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 eliminiated Echol's plans to aid Curtiss in augmenting their P-46 production capacity, 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 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.


NAA received official approval from the British Purchasing Commission on July 20th to proceed with full-scale production of the 320 NA-73 airplanes previously ordered. Four days later, the US War Department issued ‘Authority to Purchase number 165265’ for the BPC to procure 320 NA-73 airplanes plus two more to be provided to the USAAC at no cost. The two USAAC NA-73 airplanes were designated XP-51, as stated in report No. 1620.

A Radical Design

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.

The successful design of the new fighter was the work of a dedicated team (L-R): aerodynamics specialist Larry "Louis" L. Waite, NAA chief Engineer Raymond H. Rice and chief-designer: Edgar Schmued.


Once British interest in the new fighter design had become more solidified, the problem of creating a new airframe around the Allison V-1710 was the type of challenge enjoyed by Schmued. The inline V-12 required a large radiator for cooling, and its placement could greatly affect performance, both to the positive or negative.


Having a liquid cooled engine meant installing a coolant radiator. On the Spitfire, coolant radiators were installed underneath the wings. This design had 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.


Atwood  would  claim he came up  with  the idea of placing  the radiator on  the new fighter design  behind the pilot. However, Atwood's claim was rebuffed  by  several other surviving  NAA employees from the time period, who  stated  that the location was the obvious choice.

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.[2] The thrust obtainable depends upon the pressure difference between the inside and outside of the duct.[1] 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.

Meredith Effect - illustration by Steve Karp

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.


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.


The NA-73X was to have the coolant radiator installed underneath the fuselage, behind and below the cockpit, which was mainly done to preserve the streamlined design of the fighter.

 

The drawback of such an arrangement was the extra weight and combat vulnerability of the long pipes that led to and from the engine.


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.

NA-73X/P-51 Mustang Oil System
NA-73X/P-51 Mustang Oil System
Taken from P-51 Flight Manual - showing the oil tank and the lines running to the oil cooler which was installed within the radiator housing.
NA-73X/P-51 Mustang Cooling system
NA-73X/P-51 Mustang Cooling system
Taken from P-51 Flight Manual - showing the side view of the air scoop of which both the front and the back could be opened and closed. Bottom showing the engine cooling system lines running to and from the radiator.
NA-73X/P-51 Mustang Cooling system
NA-73X/P-51 Mustang Cooling system
Taken from NAA blueprints, showing a side view of the coolant lines running to and from the radiator housing
NA-73X/P-51 Mustang Radiator
NA-73X/P-51 Mustang Radiator
Taken from NAA blueprints, showing the radiator on the outside and the oil cooler on the inside

The fuselage, the smallest cross-sectional area ever to be placed be­hind an Allison engine, offered low drag but demanded high-density packing of the equipment—radios, com­passes, hydraulic systems, and the like—that had to go into it. “It’s like stuffing the insides of a bomber into a fighter,” one engineer lamented.

 

Another design feat that would ensure the NA-73X was an extremely fast fighter was the use of a brand new wing design: the laminar flow wing. Edgar Schmued recalled that, "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.”

A laminar flow wing reduces drag considerably because the wing is thinner than the previously used conventional wings.


The leading edge of the wing is more pointy and the upper and lower half of the wing are almost identical to each other. The thickest part of a laminar wing occurs at 50% chord while in the conventional design maximum thickness is at about a fifth of the way across the wing from the leading edge.


As a result, drag is significantly reduced because the wing takes considerably less effort to cut through the air. This was a very bold step especially considering the timeline promised.


On the Mustang, the smooth flow can theoretically continue more than halfway back along the airfoil before transitioning to turbulent flow, creating far less drag.

The first of the many: NA-73X

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.

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.

The competition

In this section, we will briefly explain where NAA's competitors stood at this time of the development of the Mustang.


On this page, we will discuss the progress up to mid 1941.



P-39


The Bell XP-39 was the first of NAA competitors to complete its maiden flight, on April 6th, 1938.


It achieved a top speed of 390 mph (630 km/h) at an altitude of 20,000 ft (6,100 m), reaching this altitude in only five minutes.


However, the USAAC deemed the XP-39 to be short on performance at altitude. Flight testing had found its top speed at 20,000 feet to be lower than the 400 mph claimed in the original proposal.


In June of 1939, the prototype was ordered by General Henry H. Arnold to be evaluated in NACA wind tunnels to find ways of increasing its speed, by reducing parasitic drag. NACA concluded that a top speed of 429 mph could be realized with several aerodynamic improvements they had developed and an uprated V-1710 with only a single-stage, single-speed supercharger.


In a meeting in August between the USAAC and NACA, Larry Bell stated that the production P-39 would be minus the turbocharger. Why he made this decision is a bit unclear. It could be because of Bell’s cash flow problems at that time or because of the tight and difficult installation of the engine to incorporate a turbocharger

.

The US Army eventually ordered 12 YP-39s (with only a single-stage, single-speed supercharger) for service evaluation and one YP-39A.


After these trials were complete, which resulted in detail changes including deletion of the external radiator, and on advice from NACA, the prototype was modified as the XP-39B.


After an improvement in performance was noted , the 13 YP-39s were completed to this standard, and two 0.30 in (7.62 mm) machine guns were added to the two existing 0.50 in (12.7 mm) guns.


As was the case with the P-40, the P-39 lacked any armor or self-sealing fuel tanks.


The prototype was 2,000 pounds (910 kg) lighter than the eventual production fighters.


The production P-39 retained a single-stage, single-speed supercharger with a critical altitude (above which performance declined) of about 12,000 feet (3,660 m). Without the addition of a turbocharger,  the aircraft was simpler to produce and maintain, but also deprived any chance for the aircraft to serve as a medium-high altitude front-line fighter.


When further deficiencies were noticed in 1940 and 1941, the lack of a turbo made it nearly impossible to improve upon the P-39s performance.


The original armament fit consisted of a T9 cannon with a pair of Browning M2 .50 caliber (12.7 mm) machine guns mounted in the nose. This changed to two .50 in (12.7 mm) and two .30 in (7.62 mm) guns in the XP-39B (P-39C, Model 13, the first 20 delivered) and two .50 in (12.7 mm) and four .30 in (7.62 mm) (all four in the wings) in the P-39D (Model 15).


The D-model also introduced self-sealing tanks and shackles (and piping) for a 500 lb (230 kg) bomb or drop tank.

 

Because of the P-39s unconventional layout, with the engine sitting behind the cockpit, there was no extra space in the fuselage to place a fuel tank. This meant that all of the  standard fuel load was carried in the wings, which also limited the P-39 to short-range tactical strikes.


In September 1940, the British government ordered a total of 386 P-39Ds (Model 14), with a 20 mm (.79 in) Hispano-Suza HS.404 and six .303 in (7.7 mm), instead of a 37 mm (1.46 in) cannon and six 0.30 in (7.62 mm) guns.


The RAF would eventually order a total of 675 P-39s and would name their fighter the Airacobra.

P-40


As mentioned earlier, the XP-40 was a further development of Curtiss’ H-75 Hawk design, which won a US Army fighter competition in 1935.


The prototype XP-40 was first flown on October 14, 1938.


From March 28th to April 11th, 1939, the prototype was studied by NACA. Based on the data obtained, Curtiss moved the glycol coolant radiator forward to the chin. It’s new air scoop also accommodated the oil cooler air intake.


Other improvements to the landing gear doors and the exhaust manifold combined to give performance that was satisfactory to the USAAC. Kelsey flew the XP-40 from Wright Field back to Curtiss's plant in Buffalo at an average speed of 354 mph (570 km/h). Further tests in December 1939 proved the fighter could reach 366 mph (589 km/h).


In April 1939, the U.S. Army Air Corps, having witnessed the new, sleek, high-speed, inline-engined fighters of the European air forces, placed the largest fighter order it had ever made for 524 P-40s.


The P-40 prototype was armed with one .50- and one .30-caliber machine gun, which was pretty much  the standard USAAC fighter armament during the 1930s. The production model however was armed with two .50-caliber machine guns. In keeping with President Franklin D. Roosevelt’s policy of making the latest American military hardware available to the Allies, 140 of the original batch of P-40s were diverted to France. They were armed with one .50-caliber machine gun in the fuselage and four 7.5mm guns in the wings. Because of the invasion and capitulation of France, none of those P-40s were delivered.  Instead, the export P-40s were delivered to the RAF and became known as Tomahawk Mk.Is.


The first units to convert were Hawker Hurricane squadrons of the Desert Air Force (DAF), in early 1941.


The first Tomahawks delivered came without armor, bulletproof windscreens or self-sealing fuel tanks.


Although the British were grateful for any extra combat aircraft they could get in 1940, they did not find  the Tomahawk Mk.Is suitable for combat. The P-40s still had metric instruments and other French equipment that were not compatible with RAF service, and their French throttle control levers worked in reverse of the way British or American ones didAs a result, the Tomahawk Mk.Is were relegated to tactical reconnaissance duties.


Following the first European combat experience, Curtiss started installing armor in the P-40 and increased its armament, adding a .30-caliber machine gun in each wing. The improved fighters were called P-40Bs by the Americans and Tomahawk Mk.IIs by the British. The next model, known as the P-40C, also had self-sealing fuel tanks and yet another .30-caliber machine gun in each wing.


The USAAC ordered a total of 324 P-40Bs and P-40Cs during 1941. Meanwhile, the British ordered 930 P-40Cs. Those with British radio equipment were called Tomahawk Mk.IIas, while the ones delivered to the RAF with American radios were designated Tomahawk Mk.IIbs.


First flown in April 1941, the P-40C was considered the first truly combat-ready version of the P-40 line.


Because of the added armor and armament, the gross weight increased from 7,215 to 8,058 pound, the rate of climb dropped, it became less maneuverable, and its maximum speed fell to 340 mph.


Testing by the RAF showed the aircraft did not have the performance needed for use in Northwest Europe at high-altitude, due to the service ceiling limitation. Spitfires used in the theater operated at heights around 30,000 ft (9,100 m), while the P-40's Allison engine, with its single-stage, low altitude rated supercharger, worked best at 15,000 ft (4,600 m) or lower.


When the Tomahawk was used by Allied units based in the UK from February 1941, this limitation relegated the Tomahawk to low-level reconnaissance with RAF Army Cooperation Command and only No. 403 Squadron RCAF was used in the fighter role for a mere 29 sorties, before being replaced by Spitfires. Air Ministry deemed the P-40 unsuitable for the theater.

P-38


The P-38 was first conceived in 1937 by Lockheed chief engineer Hall L. Hibbard and his then assistant, Clarence “Kelly” Johnson. The twin-boomed aircraft was the most innovative plane of its day.


Following the win of the CP39-775 competition in 1939, the XP-38 first flew on 27 January 1939 at the hands of Ben Kelsey.


Because of the speed of the XP-38, Kelsey proposed a speed dash to Wright Field on February 11th, 1939 to relocate the aircraft for further testing. General Henry "Hap" Arnold, commander of the USAAC, approved of the record attempt and recommended a cross-country flight to New York. The flight set a speed record by flying from California to New York in seven hours and two minutes, not counting two refueling stops.


On the last leg however, when Kelsey pushed the XP-38 to 420 miles per hour (680 km/h), Kelsey was ordered by Mitchel Field tower into a slow landing pattern behind other aircraft, and the prototype was downed by carburetor icing short of the Mitchel runway in Hempstead, New York, and was wrecked.


Nevertheless, on the basis of the record flight, the Air Corps ordered 13 YP-38s on April 27th, 1939 for 134,284 USD each.


Manufacture of YP-38s fell behind schedule. The 13th and final YP-38 was delivered to the Air Corps in June 1941.


Twelve aircraft were retained for flight testing and one for destructive stress testing. The YPs were substantially redesigned and differed greatly in detail from the hand-built XP-38. They were lighter and included changes in engine fit. The propeller rotation was reversed, with the blades spinning outward (away from the cockpit) at the top of their arc, rather than inward as before. This improved the aircraft's stability as a gunnery platform.

P-47


Following their CP39-770 contract bid with both the AP4-J, AP-10 and lightweight XP-47 attempt, lead designer Alexander Kartvelli took a whole different direction.


He designed a much larger fighter, named the XP-47B. The aircraft was an all-metal construction (except for the fabric-covered tail control surfaces) with elliptical wings, a straight leading edge that was slightly swept back. The air-conditioned cockpit was roomy and the pilot's seat was comfortable. The canopy doors hinged upward. Its main and auxiliary self-sealing fuel tanks were placed under the cockpit, giving a total fuel capacity of 305 US gal (254 imp gal; 1,155 l).


Biggest difference was the powerplant. Republic turned away from the inline engines and turned their attention to the huge Pratt & Whitney R-2800 Double Wasp two-row 18-cylinder radial engine, which produced 2,000 hp.


Because of negative experience with tight fit cowlings (AP-4 prototype was lost to an engine fire because of the tight fit), the huge radial engine was placed in a broad cowling that opened at the front in a "horse collar"-shaped ellipse. The huge cowling allowed for cooling the air for the engine, for cooling the left and right oil coolers, and for the turbosupercharger intercooler system.


The engine exhaust gases were routed into a pair of wastegate-equipped pipes that ran along each side of the cockpit to drive the turbosupercharger turbine at the bottom of the fuselage, about halfway between cockpit and tail. This complicated system with its ductwork gave the XP-47B its deep fuselage, ultimately leading to its nickname, “the Jug” (because its profile was similar to that of a common milk jug of the time).


Compared to the other single-engined fighters of that time, the XP-47B was a very heavy beast, with an empty weight of 9,900 lb (4,500 kg). Kartveli stated, "It will be a dinosaur, but it will be a dinosaur with good proportions.”


The armament provided was eight .50 caliber (12.7 mm) "light-barrel" Browning AN/M2 machine guns, four in each wing. The guns were installed in a staggered position to allow feeding from side-by-side ammunition boxes, carrying 350 rounds each. The combined eight guns gave the fighter a total rate of fire of approximately 100 rounds per second.


The XP-47B prototype first flew on May 6th, 1941 with Lowry P. Brabham at the controls. Despite minor issues, the aircraft proved to be impressive in its early trials.


Although the prototype was lost in an accident on August 8th, 1942, it had achieved a level speed of 412 mph (663 km/h) at 25,800 ft (7,900 m) altitude. It also demonstrated a climb from sea level to 15,000 ft (4,600 m) in five minutes.


The USAAF was impressed with the performance and placed an order for 171 production aircraft.

Specifications

NA-73X
NA-73X
NA-73X side view
NA-73X side view

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.

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.

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.” This spirit of gaining knowledge and working together were key elements in creating the NA-73X.

 

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.


Schmued began detail design on the X73 on April 21st and the NA-1620 Detail Specification was delivered to both the USAAC and the AFPB three days later (NAA project notes referred to the prototype funded by the NA-73 project as the X73, and the static-test version as XX73"). Schmued was also 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."


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.

  • Square cut wings and tail plane

  • 2 large self-sealing fuel tanks, one 90 US gallon tank in each wing, that could hold about double the fuel a Spitfire carried.

  • A wide track landing gear that would retract inward. 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 “Razorback” 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 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.


The sleek firewall forward design of NA-73X was courtesy of race pilot Art Chester, the NAA employee around which the cockpit had been designed.

 

NAA was the subject of a 27-day employee strike in June of 1941, when thousands of workers walked off their jobs shortly after midnight on Thursday, June 5th, leaving only a skeleton crew behind. The union wanted a minimum wage increase from 50 cents to 75 cents an hour, and an across-the-board 10-cents-per-hour raise for all employees. The effect of the work stoppage on the defenses of a country already engaged in war production for its allies was profound, and President Franklin Delano Roosevelt reacted swiftly. On Friday, June 6th, he signalled his plans for the government to seize the factory and force production to resume. The strike ended on July 1st.


In the beginning of July a scale model was made, using the wing, airfoil and cooling system data. The quarter-scale model was sent to GALCIT for wind tunnel testing. Between June 28th and July 26th, several issues were noted with regards to the lift and drag of the outer wing. When it was later found that his was because of the interference between the wall of the wind tunnel and the wingtips, the model was moved to a larger wind tunnel at the University of Washington in Seattle.

The tip chord of the wing was changed from 37 to 50 percent and the wingtip design was changed from round to a square tip.


After those changes the drag test results were outstanding to the NAA engineers and those dimensions were maintained.


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


The small NAA engineering department issued drawings to the fabrication shops in a never-ending stream that included 16-hour working days, seven days a week. After some 78,000 engineering man hours, first metal was cut and construction of the NA-73X prototype began.


On or about August 1st, the last of the design engineering drawings were released from Schmued’s Preliminary Design Group to the Experimental Department. It was now up to the project engineers, assisted by approximately 100 skilled draftsmen, to convert Schmued's preliminary X73 drawings into NA-73 production drawings.


As stated earlier, they did so with careful attention to both the process and fabrication of the parts and assemblies, with mass production techniques in mind. They planned ahead so that most of the hard to reach connection points and plugs for electrical and hydraulic lines and various subsystems could be assembled well before the fuselage was completed.


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


Their 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.


Dutch and his team were aiming for a delivery date of a complete airframe in January 1941.


As construction of the fuselage began, NAA requested a wooden mock-up of the Allison V-1710-39 engine in order to make sure that the engine fit properly into the tight cowling. NAA 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.


On August 9th, a request was filed by the British from the Secretary of the Treasurey for a release of 50 Allison engines. This request was almost certainly for the NA-73, but was turned down by General Hap Arnold. He defended his decision in that the USAAC was also awaiting engines and that their absence was adversely impacting pilot and crew training needs as P-38, P-39 and P-40 airframes were backing up at Lockheed, Bell and Curtiss.


NAA's B-25 prototype made its maiden flight on August 19th.


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."


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


NA-73X





Production Number


1

Dimensions



Length


32.25 ft - 9.83 m

Height


12.20 ft - 3.72 m

Width


37.04 ft - 11.28 m




Weight



Empty


6278 lbs - 2848 kg

Normal takeoff

7965 lbs - 3613 kg

Maximum gross

n/a

Powerplant

Manufacturer

Allison V-1710-F3R

CID (Cubic Inch Displacement)

1710

Normal takeoff HP

1150

War Emergency HP (WEP)

n/a

Performance

Maximum speed

382 mph - 614.77 km/h

Normal cruise speed

n/a

Climb to 20,000ft

n/a

Service ceiling

n/a

Fuel capacity

180 US gallon - 681.37 liters

Fuel capacity with drop tanks

n/a

Combat range (internal fuel)

n/a

Combat range (with drop tanks)

n/a

Armament

Machine guns

n/a

Rounds available

n/a

Bombs (lb)

n/a

Other

n/a

Serial Numbers


NA-73X





US Serial Number


NX19998

Media

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

Copyright © Christophe Haentjens - All rights reserved

Please feel free to contact us via e-mail or through the contact form on our website

Please like and follow us on Facebook!