I know it is at least once. They performed a test called Trinity.

Was it the only one?


Yes, it was the only test.

The plutonium (implosion) bomb design was only tested that one time before being used on Nagasaki. The uranium (gun) bomb design was entirely untested (*) when used on Hiroshima.

(*): "Untested" / "tested only once" refers to a complete "device" leading to a runaway chain reaction. They have been testing the heck out of the individual components, and have been conducting many tests to figure out at which point the runaway chain reaction would occur so they could build a device that would neither "fizzle" (start chain reaction too soon and go "pop" instead of "boom" because the device destroys itself prematurely) nor fail to enter chain reaction at all.

The uranium (gun) design was considered to be so simple that the chance of malfunction was minimal and did not require testing. Besides, they only had enough U235 for the one bomb anyway.

  • The "gadget" was not the actual bomb so many other items would have to be "presumed working" in order for a successful detonation by the B29 Superfortress over Hiroshima and Nagasaki. In that sense the answer to the question is an absolute no. – Doctor Zhivago Aug 8 '16 at 18:31
  • @user14394: Unclear what you are trying to say here. Trinity was the only attempt to initiate a runaway chain reaction, which is how I understood the OP's question. – DevSolar Aug 8 '16 at 18:35
  • The B29 was an experimental aircraft, the airburst concept was experimental, the method of detonation was experimental, the use of uranium was experimental...believe it or not Hiroshima was not even the original target...so "bad luck" for the Hiroshimans would appear. – Doctor Zhivago Aug 10 '16 at 1:17
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    @user14394: Sidenote: The B-29 entered operational service in May 1944. The "Silverplate" modifications to the plane were extensively tested as well, so there was nothing "experimental" about either the Enola Gay or the Bockscar. Hiroshima was the intended primary target for "Little Boy"; it was Nagasaki being the secondary target for "Fat Man" (the primary target being Kokura). I suggest you check your facts before making comments like the above. – DevSolar Aug 10 '16 at 6:54
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    @user14394: The only question I see here is the one asked by the OP, which I answered in full. I still have no idea what you are blathering about, and would thank you if you would stop. – DevSolar Aug 11 '16 at 18:38

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