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.
Sources:
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American Combat Planes, Ray Wagner, Third Enlarged Edition, Doubleday,
1982.
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The American Fighter, Enzo Angelucci and Peter Bowers, Orion Books,
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War Planes of the Second World War, Fighters, Volume Four, William
Green, Doubleday 1964.
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United States Military Aircraft since 1909, Gordon Swanborough and
Peter M. Bowers, Smithsonian, 1989.
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Fighting Mustang: The Chronicle of the P-51, William N. Hess,
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Classic Warplanes: North American P-51 Mustang, Bill Gunston, Gallery
Books, 1990.
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Famous Fighters of the Second World War, Volume I, William Green,
1967.
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The North American P-51B and C Mustang, Richard Atkins, Aircraft in
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