[SteveB]

Tony

I think there are few bits to add to your analysis.

It has been stated that the first time consideration was given to putting a Merlin in the P-51 was when Ronald Harker flew the Mustang I at AFDU on 30/4/42. As you say the first Mustang had flown in October 1940 and I believe the XP-40F had flown with a Merlin in June 1941 so it would be a surprise to find that consideration had not been given to putting a Merlin in the P-51 airframe before April 1942. Ray Wagner writes that the USAAF authorized the NAA Merlin project on 12/6/42.

The peak number of RAF Allison Mustang sqns was 16 and that number started to decline from the end of 1943 and by June 1944 there were just five Allison Mustang sqns in 2TAF. It seems that this decline in availability of Allison Mustangs was anticipated because the RAF experimented (unsuccessfully) with Typhoons for the TacR role and then after D-day introduced the Spitfire IX and finally the XIV to the TacR role. The RAF wanted either more Allison Mustangs or they wanted more Allison engines but it seems the Americans would supply neither. Yes it is correct that the A-36 and the P-51A/Mustang II kept the line going at Inglewood but it also seems to be the case that the US authorities wanted to focus all production planning on the Merlin Mustang and wanted supplies of Allison engines for other types especially the P-38.

Steve