See the sabotage of Norwegian heavy water:
https://en.wikipedia.org/wiki/Norwegian_heavy_water_sabotage
Plausible only for those without knowledge of the extent of the US program. Even among those involved, few had perspective on exactly how much of the US economy was being funnelled into the nuclear program. The Manhattan Project famously at some points used roughly 10% of the electricity available in the united states, a country with an infrastructure relatively untouched by war. There is absolutely no way that WWII Germany could have sustained anything close to a similar effort. They had to brains to understand the physics, but not the raw industrial power necessary to turn that into a practical weapon.
The Japanese had two different atomic bomb projects, as usual, one for the navy and one for the army. As I understand it, one of the (the navy's) got pretty close. But details of the Japanese effort are hard to find.
A much more focused project that consumed a smaller portion of Europe's industrial capacity was somewhat plausible, at least up until the scuttling of Norway's heavy water. A serious German bomb effort could have done the minimum uranium enrichment (using centrifuges, perhaps) needed to breed plutonium in a reactor, and then yielded plutonium bombs, at much lower cost than the Manhattan project. Necessity being the mother of invention, and with all the energetic and industrial constraints they had it's not too farfetched to suppose they would have landed an idea like that. Still, once they lost access to Norway's heavy water, it would have cost them a great deal of electric power to make more, so there's that.
Which also serves to highlight the incredible cost of the B-29 project — $1.9 billion for the Manhattan project, $3 billion for the B-29.