P-51 D/K (Mustang IV)

Cadillac of the skies

Design changes

Production & K-model

Later modifications

Operational service

The legend lives on


Serial numbers

3-Way drawings



Cadillac of the skies

The P-51B and C models were excellent fighters that served until the end of war but they still had many deficiencies. NAA engineers were quick to recognize them and by the end of 1943 were already working on the next variant of the breed: the P-51D.

With the P-51D version, the USAAF finally had the air superiority and Luftwaffe resistance began to fade. With the Merlin, the Mustang could fly for over 8 hours and it could accompany the bombers to their targets and back. Additionaly, they strafed airfields and targets of opportunity on the way home.

The D-model was and still is the best known Mustang model and over 8,102 were produced.


Design changes

The P-51D was designed to remedy the shortcomings and complaints that were still existing with the P-51B/C models (such as poor rearward visibility, gun jamming and lack of firepower) and to make other improvements to the Mustang design. These changes included:


  • A bubble canopy:
    Even the installation of the “Malcolm Hood” did not really fix the rearward visibility problems. In 1943, the technology to create distortion free large curved plexiglass was developed and the British figured out how to make a single-piece bubble canopy.
    This bubble (or “teardrop”) canopy offered an unobstructed 360° view and was used on British Typhoons and Spitfires. Col. Mark Bradley visited England in 1943 and had seen the new bubble canopy being developed by the British. He returned to the US and lobbied to get the new bubble canopy on US fighters.
    The first US fighter to incorporate the new canopy was Republic's P-47 Thunderbolt. Bradley flew the modified Thunderbolt to California and showed it to NAA's Dutch Kindleberger.
    One of the early P-51B-1-NA Mustangs was chosen for initial modifications. NAA engineers found a way to cut down the rear “Razorback” fuselage and mount the canopy over the cockpit. To provide strength and stability and to avoid distortion or jamming of the canopy, it was mounted in a very deep, rubber-lined metal frame. The canopy was opened and closed manually by a handle crank operated by the pilot and slid back over three rails (one along each side of the canopy and one along the centerline of the rear fuselage).
    Courtesy of In Action 45 - P-51 Mustang by Squadron/Signal Publications
  • New and heavier armament:
    The four 0.50 caliber machine guns on the B & C-model Mustangs proved to be limited for certain tasks, but more importantly they had the tendancy of jamming during high-G maneuvers.
    With the D-model the wings were remodeled to fit a total of six 0.50 caliber Browning machine guns, three in each wing. Each gun was now also mounted in an upright position and as a result could be fed by non-kinked, power fed ammunition belts.
    On each wing, the inner gun carried 400 rounds and was set back of the outer guns. The two outer guns had 280 rounds each and were mounted side by side. The ammo boxes were stacked in the wing outboard of the guns with the box for the outer gun on the bottom and the box for the middle gun on top. The belt feed for the middle arched over the outer gun.
    The option remained to remove both middle guns and to have just four guns with 400 rounds each.
    Courtesy of In Action 45 - P-51 Mustang by Squadron/Signal Publications

    Photo by Christophe Haentjens
  • The landing gear was made stronger to handle the increased weight and gear operation was changed. The wheel bays and doors were changed. These changes, along with the change in armament resulted in a slightly larger wing chord and a “kink” in the leading edge of the wing near the wing root.
  • The K-14 gun sight:
    A significant improvement in the D-model was not structural and would very much improve the Mustangs lethality. The British had been working on a Ferranti “GGS Mark IID” computed gyro gun sight. This was manufactured under license in the US as the K-14 gun sight and was first used in October of 1944 in the -20NA blocks.
    The K-14 gun sight used an analog computer and allowed the pilot to dial in the target wingspan (on the gun sight) and range (by turning a handgrip on the throttle lever). It then computed and displayed the correct angle of deflection needed to hit the target and thus greatly improved the accuracy in deflection shots.

    Above: difference between the provious N-9 model gun sight and the new K-14 - Below: the new K-14 gun sight
    K-14 gunsight
  • Aileron effectiveness and strength were also improved by adding a seal to the leading edge
  • The landing light was moved from the leading edge of the wing to a position inside the gear well. As the gear extended so would the landing light
  • Later P-51Ds would also have an AN/APS-13 tail warning radar with rod antennas sprouting from the vertical stabilizer. These gave warning to the pilot of an incoming attack from the rear.
    Courtesy of In Action 45 - P-51 Mustang by Squadron/Signal Publications

NAA pulled P-51B-1NA 43-12102 from the production line to begin modifications for the D-model. The rear fuselage was cut down and a bubble canopy was installed (no other modifications that appeared on production D-models were made and it did not have the 85 gallon fuselage tank). This aircraft is often referred to as XP-51D, but this designation was never officially used. It was first flown by test pilot Bob Chilton on November 17th, 1943, and received a big thumbs up.

P-51B-1NA 43-12102 (USAF)
P-51B-1NA 43-12102, also referred to as XP-51D with the bubble canopy and cut down razorback, the rest of the airframe was still standard B/C-model (the photo clearly shows the B/C-model wing)

With this modification being successful, two more P-51Bs were pulled from the assembly line (42-106539 and 42-106540) and were given the NAA designation NA-106. They had the full armament installed.

When all modifications were made, the P-51D production line (designated NA-109) was started.

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