The North American Aviation P-51 Mustang was an American long-range, single-seat fighter and fighter-bomber used during World War II, the Korean War and other conflicts. The Mustang was conceived, designed and built by North American Aviation (NAA) in response to a specification issued directly to NAA by the British Purchasing Commission. The prototype NA-73X airframe was rolled out on 9 September 1940, 102 days after the contract was signed and, with an engine installed, first flew on 26 October. The Mustang was originally designed to use the Allison V-1710 engine, which had limited high-altitude performance. It was first flown operationally by the Royal Air Force (RAF) as a tactical-reconnaissance aircraft and fighter-bomber (Mustang Mk I). The addition of the Rolls-Royce Merlin to the P-51B/C model transformed the Mustangs performance at altitudes above 15,000 ft, giving it a much better performance that matched or bettered almost all of the Luftwaffes fighters at altitude. The definitive version, the P-51D, was powered by the Packard V-1650-7, a license-built version of the Rolls-Royce Merlin 60 series two-stage two-speed supercharged engine, and armed with six .50 caliber(12.7 mm) M2 Browning machine guns.