Another option to consider might be that an earlier entry into the war (including the presumed lead up) triggers an earlier deal to produce the Packard V-1650 Merlin. In the real world an agreement was reached between Rolls-Royce and the Packard Motor Car Company in September 1940 to manufacture the Merlin under license, with the first Packard-built engine, designated V-1650-1, run in August 1941. If the USA were to enter the war in Sep 1939, alongside the UK and France and others, perhaps a deal could have been done in late 1939, thus bringing the Packard Merlin forward by close to a year thus seeing the first engines run in late 1940. This then could have the flow on effect of bringing something such as the P-40F (Kittyhawk Mk II) into service in late 1940 and maybe, just maybe, see them participate in the Battle of Britain.
