By 1942 no one really had doubts about that. I mean, the Einstein–Szilard letter was written in 1939 and Bohr mentioned in lectures also in 1939 the bomb is possible but he didn't think it was possible to produce enough weapons grade uranium within the timeframe of the war. He couldn't foresee, no one could, really, what the US could do when determined.
These details were covered in depth in The Making of the Atomic Bomb by Richard Rhodes. The Oppenheimer movie glosses over a lot of the technical details. It’s worth a read.
