The P-51 Mustang is an American long-range, single-seat fighter and fighter-bomber used during World War II and the Korean War, among other conflicts. The Mustang was designed in 1940 in response to a requirement of the British Purchasing Commission, the original intention was to build Curtiss P-40 fighters under license for the Royal Air Force though a brand new design of a more modern fighter was proposed and produced. The prototype NA-73X airframe was rolled out on 9 September 1940, 102 days after the contract was signed, and first flew on 26 October.
The Mustang was designed to use the Allison V-1710 engine which, in its earlier variants, had limited high-altitude performance. The aircraft was first flown operationally by the RAF as a tactical-reconnaissance aircraft and fighter-bomber (Mustang Mk I). Replacing the Allison with a Rolls-Royce Merlin resulted in the P-51B/C (Mustang Mk III) model and transformed the aircraft’s performance at altitudes above 15,000 ft (4,600 m) (without sacrificing range), allowing it to compete with the Luftwaffe’s fighters. The definitive version, the P-51D, was powered by the Packard V-1650-7, a license-built version of the two-speed two-stage-supercharged Merlin 66, and was armed with six .50 caliber (12.7 mm) M2/AN Browning machine guns.