The P-51 destroyed more enemy aircraft than any other fighter in Europe. It began as the NA-73 in 1940 at Britain’s request. The design showed promise and the Army Air Forces purchases of Allison-powered Mustangs began in 1941 primarily for photo reconnaissance and ground support use due to its limited high-altitude performance. In 1942, tests of P-51s using the British Rolls-Royce “Merlin” engine revealed much improved speed and service ceiling, and in December 1943, Merlin-powered P-51Bs first entered combat over Europe. Providing high-altitude escort to B-17s and B-24s, they scored heavily over German interceptors and by war’s end, P-51s had destroyed 4,950 enemy aircraft in the air, more than any other fighter.