“It will never go off. And I speak as an expert on explosives.”
Admiral Richard Leahy, USN, on the Atomic bomb, 1945
He probably was an expert on chemical explosives. But many of the scientists weren’t sure it would work either because they couldn’t build a device to harness the nuclear physics reactions.
Apparently some of the scientists believed that first bomb going off would cause a worldwide nuclear reaction destroying the whole planet . They had bets going on the outcome.