I have to say that the B-17 is most important american bomber.
Fixed. It's close-minded to think the B-17 is the most important bomber of the war, when Halifaxes and Wellingtons made up the bulk of the allied bomber fleet for half the war. They still had Wellingtons flying in 1000-plane missions in 1944, as commented by at least 1 Luftwaffe pilot making note "many were the older types, Wellingtons and so forth" [paraphrase]
Also, the G4M betty may well be the most important, with its roles all over the CBI, the PTO, and anywhere the Japanese operated. Or you might say the Lancaster was the shining light for the RAF Bomber command, and was the iconic and quintessential bomber of the war. Or you might say the Ju-88 served in far greater capacity and in a far wider variety of roles and in greater levels of evolution than the B-17 ever did.
So please keep in mind the U.S.A. wasn't the only country in the war, and we only joined it halfway through.
P.S. Foget the blasted PBY. It's a POS with fabric covered wings and no redeeming values. It has no place in AH and no role in AH's gameplay.