The XP51 was a pure NAA design. British Purchasing Commission
approached NAA in 1939 about building P40s. NAA replied they could
build a better fighter. NAA had been designing opne for some time. The
design was firmed up in April 1940; the contract with the Bitish was
signed in May, 1940. Delivery was set for January 1941. The aircraft
minus engine was ready 18 days early. The engine was 20 days late
because of priorities and because Allison didn't think NAA would be
ready. First fight of the NA-73 wa in October 1940. NAA started mass
production for the British. December, 1940, NAA received a letter from
the British Purchasing Commission informing NAA that the airplane was
named "Mustang". Some Mustang 1s were armed with 4x20mm. RAF used them
for low-altitude missions.
Wright Field had 2 XP51s (named 'Apache') that were ignored for a time
until NAA finished their production run for the RAF and Arnold
realized here was a fighter production line - now idle. Thus NAA got a
contract for 500 A36s - P51As with dive brakes. They were for the
USAAC in the Med where something better than the P40 was needed for
CAS/interdiction.
Meanwhile the USAAC military attache in London had flown a Mustang 1
and he, with some high-ranking RAF types, lobbied for the Merlin
installation. First flight with a Merlin was in October 1942 - the
rest is history. Also, FWIW, the first American-made Merlin ran in May
1941
Note: one can build a Mustang from the original data - every necessary
data point can be established in space using direction cosines. I do
not know of any other airplane for which this data/capability exists.
This info from "Pursue and Destroy", by Major L.K. Carson, who after
his war service became part of the test staff at Wright-Patterson AFB.
Excellent book. ISBN 0-913194-05-0
Walt BJ