Alt USAAC/AAF fighters, 1937-45

[tomo pauk]

This may well be so, however I would gently point out that the P-51 did not make any real contribution to the European air war until almost four years after it was designed. So, as Lusser points out, one can rush things, but ultimately, despite what one might wish, four years is how long it will REALLY take to mature the concept until its actually operationally useful.

Let's look at it from the POV of the USAAF in 1940-1941...
The 1940 fighter program's covered by Curtiss with their P-40 (July 1940 onwards).
The 1941 fighter program's covered by Lockheed with their P-38 (July 1941 onwards) and Bell with their P-39 (June 1941 onwards).
For the 1942 fighter program, we've got the wunderplane P-47, capable of flying at extremely high altitudes and fast speeds coming sometime in mid-1942 (with the P-43 being built in 1941 even though it's obsolete to keep Republic in business); and maybe the P-63 as a backup for late 1942, early 1943.
For 1943+ onwards, we've got the R-40C program (whatever may come of it).

So, how do we slot the Mustang into the AAF production program?
The answer is...we can't.
Not when North American is working on:
AT-6 Texan (we kind of need this to train the thousands of pilots for the thousands of planes FDR wants)
B-25 Mitchell (to equip the 1940-41 medium bomber program)
B-28 (to equip the 1942-1943 high altitude medium bomber program).

I reckoned that talk about USAAC/AAF alternative fighters is better done in it's own thread
A few things that seem worthy of a slight disagreement:
- perhaps saying 'budget for fighters in XYZ year' might be more accurate to say, than the 1940 or 1941 fighter program
- P-43 obsolete in 1941/42 - that depends on with what it is compared, eg. it was a far better fighter above 15000 ft than the P-40E or P-39D
- P-63: if that aircraft is worth manufacturing, the P-51 is even more deserving, and it actually is flying a full year before the P-63
- the P-51 is an actually available military hardware well before the Curtiss P-60 program can materialize (didn't happened in the end), yet USAAF found that worthy of financing

So there is a lot of elbow room for the P-51, unfortunately USAAF was oblivious to it by many months. NAA manufactured 500 A-36s - pointing out that production capacity was there, just it took budgetary manuevering ("we have the budget for attack A/C, but not for fighters, so make the A-36s instead of P-51s") for the NAA-73 lineage not to go extinct.

More to the point (and this is REALLY important) was that the Mustang had not been built to AAF specifications -- it was basically built "out of the system", to British/Commercial specifications, so there was a strong institutional distrust of the Mustang as it was not invented here (NIH).
[Back then, there was always a problem of "cranks" claiming that their idea was a world beater, if only the HIDEBOUND CONSERVATIVES WOULD SEE IT.]

The USAAF went full bore NIH in this case. It took them until 1st March of 1942 to start testing the XP-51 that was delivered to Wright Field in 24th August 1941 - a six month delay. (source: "American's hundred thousand" book pg. 330)
Looks pretty hidebound to me.

It took time for the AAF's "next generation" fighter programs to fail as miserably as everyone else's (Look at Germany's Me 109/209/309 and Me 210/410) before people started remembering the spectacular range and performance of the Allison engined Mustangs in flight test.
The P-51A with 2 x 75 gallon drop tanks had a combat radius of about 600 miles due to it's extremely low drag versus other fighters.
So if the Allison Mustang could fly to Berlin and back, why the switch to the Merlin, imposing about half a years' delay?
Because by the time people had finally circled back to the Mustang as the solution to the AAF's problem(s), the threat had advanced -- no longer was the likely threat B-17 type aircraft...but now B-29 type aircraft.
Additionally, theoretical enemy fighter capabilities had increased -- in order to be capable of intercepting a B-29 class target.
Only the Merlin had a working two-stage supercharger -- the Allison two-stage supercharger was too immature for production.

P-51A have had a splendid combat radius due to a combination of huge fuel load, frugal engine, and indeed low drag. What P-51A didn't have was timing - 1st delivered in March 1943, and only 310 produced; 1st combat was in late 1943. What was also lacking was performance at 25000+- ft (altitude band where the B-17s were cruising against Germany). That alone was a good reason for not trying to use them for the escort work over Germany proper, and a major reason why the 2-stage Merlin ended up in the Mustangs.
V-1710 with 2-stage S/C was used on P-61; granted, not many of these were produced in 1943.

With 4 .50s, probably nobody was expecting that P-51B will be a proper tool to intercept the B-29 class target.