Who developed the atomic bomb?

Answer 1

The US Army's Manhattan Project under the military leadership of General Leslie Groves and the scientific directorship of Robert Oppenheimer (the two men in the image above). The project spent two billion dollars, mostly on building factories and other atomic infrastructure, and employed more than 130,000 people at sites scattered across the US.

It is a common misconception that Albert Einstein was part of the Manhattan Project. As a pacifist he refused to perform any war related work and the US Army security would never have granted him the necessary security clearances (they refused to grant Robert Oppenheimer these clearances and it took a direct order from General Groves to override this and get Oppenheimer cleared as Groves believed he was the only scientist that Groves could communicate with about the issues needed to make the project succeed).

The only participation Einstein had was to sign a letter on August 2, 1939 previously written by Leo Szilard (who invented the atomic bomb in 1933 while in London, patent GB630726) addressed to president Franklin Roosevelt. After the letter was sent he knew nothing of whether the US was even studying it or not until he saw the August 6, 1945 newspaper headlines about the atomic bombing of Hiroshima that morning.