Maybe there would be a move to develop a Lycoming or license-built RR, BMW or DB V-12 modification of the P-36 that resulted in license production of a foreign engine? Weren't the initial Bf-109s powered by RR engines? So once the idea of a P-36 with a liquid cooled engine is floated, the provider becomes who ever is building a possible engine, no?
The Lycoming engine was too small (not to the Army's eyes
at the time) and as it existed was a "flat" engine, it could have been been bent into a "V" but that adds a bit more time. It was a 1234 cu un ( 20.2 L) engine, smaller than a Kestrel or Peregrine and depended on high rpm and high boost to make it's power. It used a short stroke so the RPM may not have been a big problem.
The German engines are pretty much non-starters unless you really push history, Not so much from the "Not invented here" stand point but from the fact that the DB 601 was pretty much "vapor ware) as far as the rest of the world was concerned in 1938. At the Paris Air show/exhibition in Dec 1938 the DB 601 was talked about but NOT displayed. RR had a Merlin X (two speed supercharger) on display and was talking about hp figures using 100 octane fuel. Information on large German engines was sketchy, to say the least. People to not take out licences on engines they don't know about. The Only BMW V12s were the old one that was pretty much two WW I straight sixes on a common crankshaft and a small experimental built to compete with the Jumo 210.
A single 109 was powered by a RR Kestrel because the Jumo 210 had not been cleared for flight at the time. This one Kestrel may have made the "rounds" and powered not only the first JU 87 but another prototype as well.
Alternatively, the army would have a variety of radial development options although it might have resulted in an odd mix of a/c operational on December 7, 1941. The US army could have subjected to further development and ultimately bought more upgraded P-36s and P-43s and perhaps land based F4F-3s or P-66s by December 7th, 1941 with perhaps a strictly land-based USAAF F4U variant in the wings or instead of the P38 perhaps the USAAF might push development of the Grumman XP-50 (first flight 2/18/41).
The upgraded P-36 is a possibility but it is still well behind the world standard. Production P-36s had single speed superchargers and about 1050hp for take-off. Later (1941) R-1830s got 2 speed superchargers and other improvements and not only had 1200hp for take off but were allowed 1200hp at 5,000ft, and 1050 at 13,100ft Military ratings. Any improvement after that had to wait several more years. Vought built 178 F4us in 1942, and Land Based modification is a non-starter. Land based F4Fs and P-66s hit the engine problem too. You can't take enough weight out of the F4F to make a real difference without redesigning the whole thing and P W had trouble making enough two stage engines as it was which is why there was a F4F-3A model with two speed SINGLE stage supercharger whose performance was a little less than thrilling at altitude. FTH was 5-6,000 ft lower than than the F4F-3.
US might push development and production of the XP-50 but since it was destroyed early in the test program it's published numbers must be looked at with a fair degree of suspicion as the XF5F missed it's estimated numbers by a fair margin. US had problems with turbo controls on ALL it's turbo charged aircraft in the early part of the war.
P-43s come ONLY at the expense of the P-47 program. How big an expense depends on how and when you play with the program. Sept 13 1940 had the US Army place an order for 773 P-47s off the drawing board
after they decide the proposed (and mocked up) liquid cooled P-47A is a non-starter.
I would expect the state of the US a/c industry would have inspired the BPC to go around to a number of manufactures asking them to produce something, anything but what they were currently building for the USAAF and the P-51B would have been born earlier or maybe USA built Hurricanes or Spitfires or lots and lots of Boulton Paul Defiants. Or best of all, the army might have obtained thousands of Bell FM-1s aerocudas.
The BPC was limited as to what they could order. The US had gotten burned in WW I when it finally declared war and found it's factories were tooled up for an assortment of foreign weapons it didn't want and had a hard time using. IF the US PTB (powers that be) thought a foreign design was acceptable to the US Military it would be given the OK for production, if not then no production. No US company made a British tank design and the US had squat for tanks at the time. There was a set of standards for US military aircraft at the time that covered a bunch of things
aside from the actual performance numbers, standards for stress loadings, landing impacts, construction details (fittings and such). The US did not what a bunch of factories tooled up and producing Whitworth nuts, bolts, fittings and such.
For all its faults, I don't think the world is necessarily better off without the Allison.
I think that is a little harsh, the
only real fault the Allison had was the lack of a really good supercharger between late 1940 and 1943/44. It's supercharger was no worse than the German ones in 1940-41-42. The Germans just didn't try to power 7,500-8,500lb planes with 1200hp engines. Blaming the engine doesn't seem quite fair.