XP-78 was the designation initially given to a version of the North American P-51 Mustang powered by the Packard-built Rolls Royce Merlin liquid-cooled engine. On July 25, 1942, North American Aviation was authorized to adapt two P-51 Mustang airframes to Merlin 65 engines imported from England. For the conversion, NAA selected two P-51s from the batch of Mustang IAs that had been repossessed from the RAF by the USAAF. Their serial numbers were 41-37350 and 41-37421. NAA gave the project the company designation NA-101. These aircraft were considered sufficiently different from the existing Mustang that they were given a new designation--XP-78. However, the designation of these two aircraft was changed to XP-51B while the work was progressing.