December 1943, The P-51 was designed as the NA-73 in 1940 at Britain's request. The design showed promise and Army Air Forces (AAF) purchases of Allison-powered Mustangs began in 1941, primarily for photo reconnaissance and ground support use due to its limited high-altitude performance. But 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-51As first entered combat over Europe.