The prototype for Fat Man (plutonium bomb) was detonated at Almogordo, NM on July 16, 1945.
It worked, so it was a pretty sure thing that Fat Man would work also.
Little Boy (uranium bomb) made it’s debut over Hiroshima; that design was never tested previously. Obviously, it worked just fine.
Which one did they stick with before the hydrogen bomb? Or did they still make both plutonium and uranium bombs?