It is indeed nonsense to say the the P-51 was developed from the XP-46!
Originally, the Mustang, or NA-73X was not even developed for the AAC, but for the RAF!
North American produced the NA-73X prototype on the request of the Britihs Purchase Commission when they were short of fighters in 1940! Even when it was already flown, the AAC was not interested in this type until it was evaluated at Wright Field showing its highly efficient airframe. And even then it was not ordered as a fighter but as a ground-attack plane!
With the original Allison V-1710 its performances detoriated very fast at higher altitude and it was not until the Mustang airframe was mated with a Rolls Royce Merlin when it showed its real potential. And..........the Merlin was fitted for the first time by the British to a UK supplied Mustang!
Fitted with a Merlin, a bubble canopy and an extra 1200 l fuel tank behind the cockpit it became the great fighter it is now claimed to be! It is ironical that the original AAC specifications for a long-range escort fighter only produced big monsters like the XP-67 and the P-75A that were totally unsuitable while the Mustang was already flying around for some time..........
