This site says Hap Arnold was so convinced bombers could outrun fighters, & didn't need escorts, he actually banned development of a 52USgal tank in Feb '39, suitable for the P-36, claiming it was unsafe. (It seems Arnold wanted a 250pd bomb, instead...)
It goes on to say field improvisations, including one by Kenney's Fifth using Spam cans,


showed what could be done--but AAF Materiel Command was too busy buggering around, & Arnold too obsessed with bombers, for drop tanks to see production for more than 20mo.



(In the event, Kenney's Spamtanks were better.


)
That all being true, what would have been the effect had P-38s, P-40s, & (later) P-47s started the war with drop tanks? It appears it would have enabled P-38s to reach well into Germany, possibly all the way to Berlin:
This would appear to butterfly away the P-51...

What other influences might it have had?