I should also add that the RAF saved Britain's "ass" in the late summer and fall of 1940, when they (just barely) held off the German Luftwaffe in the Battle of Britain. It was a VERY close thing, could have gone either way but for one or two bad German decisions, but the British did it by themselves (with the aid of some very brave Polish, Czech, French, and Canadian pilots who also served in the RAF during that campaign. The USA played no part in that.

One thing the USA did that was quite helpful to Britain in the dark days of the early war was to supply about 50 old American destroyers to the Royal Navy as part of the Lend Lease Act of 1940. This did help the British significantly. It didn't win the war, but it helped. Credit where credit is due. The full story of that is here:

https://en.wikipedia.org/wiki/Destroyers_for_Bases_Agreement