H. G. Wells used the Words "atomic bomb" in his 1914 novel The World Set Free.
Leo Szilard filed the first atomic bomb related patent in 1934, on the neutron chain reaction.
A joint German and Swedish team discovered nuclear fission in uranium-235 in 1939.
The US Manhattan Project developed the infrastructure to build atomic bombs, beginning production in 1945. This project employed tens of thousands of workers.
