Who thought of the idea to build an atomic bomb?

Einstein has some credit in the development of the nuclear bomb, but never thought of it himself... I believe it was Hitler who first thought of the atomic bomb. The Germans were working on a way to harness the energy from a split atom. Einstein sent the US president a letter, telling him of the power such a weapon could have, and the Germans were working on splitting atoms. That brought about the Manhattan Project.

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The Einstein letter to FDR was actually entirely written by Leo Szilard, all Einstein did was sign it.

The writer H.G. Wells had imagined them and wrote about them in his 1914 novel "The World Set Free", but this was just SciFi.

Leo Szilard was the first to actually invent the process that makes the atomic bomb work (neutron chain reaction) in 1933 while in London and patented this on June 28, 1934.

A team of German scientists working in Germany and Sweden discovered fission in uranium-235 in 1939 and after the publication of their paper, Germany blocked further publication of the results of all research in nuclear physics.

By the time the Manhattan Project became fully active, Germany had already decided to scale back their nuclear project to just basic reactor research with the single goal of building power reactors after Germany won the war. They considered a bomb impractical in the time estimated to win the war by conventional means, even if it was possible (and Heisenberg's calculations suggested such a bomb would be too large for any available aircraft to deliver).

To Hitler the idea of an Atomic Bomb sounded like Jewish Physics, and thus nonsense. Hitler knew nothing of physics, nuclear or otherwise, and the teaching of Einstein's theories of relativity were already banned in Nazi Germany simply because Einstein was a Jew (implicitly making it Jewish Physics) regardless of any experimental evidence confirming the theories.