North American P-51 Mustang XP-51D

The XP-51D was not a new design but a set of conversions of existing P-51B airframes, intended to cure the most criticised flaw of the Merlin-engined "razorback" Mustang: poor visibility to the rear. The P-51B had grown out of the XP-51B, which North American test pilot Bob Chilton first flew on 30 November 1942, and it entered production at North American's Inglewood, California, plant in June 1943. Its framed, hinged canopy sat flush with a high rear spine. British pilots solved the problem pragmatically by fitting the bulged plexiglass Malcolm hood also used on the Spitfire, a modification later adopted by many American P-51B/Cs of the Eighth Air Force.

The American answer went further. According to the French account, Colonel Mark Bradley of the US Army Air Forces returned from a trip to England, where he had seen the new Spitfires and Hawker Typhoons with "teardrop" canopies, in June 1943, determined to see the feature adopted by American manufacturers. Republic adapted its P-47D in record time and Bradley showed the result to Kindelberger at North American. An arrangement was then made with the Army to try the modification on a selected P-51B, which took the name XP-51D.

The aircraft chosen, according to the English-language variants history, was the tenth production P-51B-1-NA, serial 43-12102, taken off the assembly line. From the windscreen aft, its rear fuselage formers were cut down to the same height as those forward of the cockpit, and the new line was faired into the vertical tail; the French source adds that the lowered rear fuselage was strengthened accordingly. A simpler windscreen, with an angled bullet-resistant panel mounted on two flat side pieces, improved the forward view. The new canopy drew on moulding techniques developed in the United States for the streamlined nose transparencies of bombers: a large blown plexiglass hood in a metal frame with a rubber seal, running on three rails so that the pilot could slide it aft with a hand crank. Its shape was fixed after wind-tunnel testing to obtain the best compromise between drag and field of view, with the highest point well behind the pilot's head; tests of a wooden model had confirmed that the aerodynamics were sound. The result gave exceptional all-round visibility.

The French account dates the first flight of the converted aircraft to 17 November 1943, again with Bob Chilton at the controls. With the concept validated, North American pulled two P-51B-10-NA airframes from the assembly line and completed them with the new canopy as P-51Ds under the company designation NA-106. The opportunity was taken to fix the armament: on the B the guns were mounted lying on their side and tended to jam in hard manoeuvres, so three new guns were installed upright in each wing, for six 12.7 mm machine guns in all, with 400 rounds for the two central guns and 270 for the others; pilots could have the outer pair removed, leaving four guns with 400 rounds each.

The sources do not agree on how many aircraft the name covers or on the timeline. The English variants list describes the XP-51D (NA-106) as one P-51B-1-NA and two B-10-NAs, modified and tested with a cut-down rear fuselage and clear-blown canopy, adding that the XP-51D designation was later changed to P-51D-NA; the Spanish list likewise counts three P-51D prototypes. The French article reserves the name XP-51D for the single selected P-51B and calls the two B-10s P-51D (NA-106), while the German table records only two converted P-51Bs with the full-vision canopy and six 12.7 mm guns. The English text also places the removal of the tenth P-51B-1 in late 1942, whereas the French narrative only begins with Bradley's return in June 1943. The engine is not given for each airframe: the P-51B-1-NA block used the Packard V-1650-3, and the B-10-NA switched to the V-1650-7 late in its run.

What these airframes proved became the production P-51D. The new model also received a redesigned wing: changes to the undercarriage up-locks and inner-door mechanisms added a fillet forward of each wheel bay, creating a distinctive kink at the wing root leading edge, and the extra weight required a strengthened undercarriage with larger main tyres. The change was not instantaneous: the first four aircraft of the initial P-51D-1-NA block, sent unassembled to Australia, reportedly still had the razorback canopy of the P-51B/C. The sources consulted give no individual performance figures, precise dates for the other conversions, or fates for these aircraft.