The big problem seems to be the making of U-235, which both Germany and Japan were both doing the wrong process and also did not have enough material to make the bomb. Also Plutonium was one possibility but it seems only the US team knew of its existence.

And just think that the Manhattan project cost a bit...where could Germany or Japan get that kinda resources truly?!

http://www.brook.edu/FP/PROJECTS/NUCWCOST/MANHATTN.HTM

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http://search.japantimes.co.jp/membe...20030307b7.htm

WARTIME DOCUMENTS SET RECORD STRAIGHT
Japan's A-bomb goal still long way off in '45

"The documents are one of a kind. We can finally prove that even if Japan had built a bomb, it would not have been powerful at all," said Masakatsu Yamazaki, a professor of science history at the Tokyo Institute of Technology who analyzed the papers. "And it might have taken them another decade to complete one."

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No wonder the Japanese had a project or more on A-bomb. The Germans seems to have had three or four different projects going on towards the end of war, Luftwaffe and Waffen-SS had their own ones, too. And none of the teams knew what the other one was doing (!)

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