I've finished https://www.goodreads.com/book/show/170428.Now_It_Can_Be_Tol... last week, and from what I understand, a lot of people didn't think that bomb is a possibility (for example, the Germans weren't really sure), and even when the Manhattan project was running several years, the scientists still weren't sure that the bomb will work as expected. There was a lot of variables in play, and for example, materials for the bombs weren't available for testing in large enough qualities.

One of the concerns, from what I understand as a layman, was that a critical amount of material will blow itself up too early, so that most of the material won't be part of the explosion. This was solved, I think, by some kind of neutron reflectors, or something*.

There are few chapters about how part of the Manhattan project was capturing scientists from France, Italy and Germany. When they captured them, their housing was bugged and there are transcripts from that time in the book. When the bombs were dropped, they generally couldn't believe that it really worked.

* One of the transcripts shows, that the Germans didn't think of this, and when they thought about the bomb, the thought about basically overloaded nuclear reactor, which would be big, heavy and impractical for use as a bomb.