The Allison engine powered P-51A Apache, was derived from the original RAF Mustang 1 ground attack and photo reconnaissance aircraft. Long before anyone had even heard of a Mustang, the plan to put a Rolls Royce Merlin engine into the airframe had started. The Merlin was powering some of the best warplanes in England; the Hurricane, Spitfire and the Avro Lancaster bomber. To develop a new engine for the P-51 would take considerable time, which the allies did not have.
In September 1940, the Packard Motor Company of the U.S. began license production of the Merlin V-1650-1 engine in Detroit, Michigan. With a new supercharger drive, a new V-1650-3 was mated to the first P-51B. Both the U.S. and England started the Merlin engine modification idea almost simultaneously. 10 airframes were modified in the U.S. with the Merlin and these became the basis for the most successful fighter series of the war.