And the winner is ....... 8028410q. Well done
XP-51 No. 4 is in the EAA museum. Details here including a very nice interactive cockpit display:
North American XP-51 Mustang
Perhaps someone can shed light on the conficting details on the web. The EAA say that their example was the fourth P-51 built. Elsewhere I've seen the claim that the prototype was the NA-73X and that XP-51 was the designation for aircraft from the first production batch of Mustang 1s that were diverted to the USAAC for evaluation.
The MAP order was placed on 26 June 1940. The prototype first flew on 20 October. Less than four months! A remarkable beginning for a remarkable aircraft.
I've been unable to find a reference I saw some time ago as to how the EAA obtained the XP-51. The gist is that the EAA gave the NASM an aircraft that the museum wanted and in return was told to in effect, "take one of those containers in the storage area". The EAA rep saw the magic word "XP-51" and had it signed out and on a truck before the museum realized what they had given away!