As it says on the tin.
What would be the effects on the Allied air campaigns in WWII if there was an earlier development of North American's P-51 Series, basically the aircraft being developed around the early 1930's, the same time as the Hurricane, Spitfire and Bf 109?
What would be the effects of these aircraft being in service from 1939?
What effect would it have on the Luftwaffe or even the Japanese?
How would the BoB or even Battle of France play out if either were used by the RAF or the French?
Would they be used as part of 'Lend-Lease' to the USSR?