P-51D Mustang "Belligerent Bets"The P 51D Mustang was designed as the NA 73 in 1940 at Britain's request. The design showed promise and AAF purchases of Allison powered Mustangs began in 1941 primarily for photo recon 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 Dec. 1943, Merlin powered P 51Bs first entered combat over Europe.