Atomic bombs were codenamed 'Little Boy' and 'Fat Man'

The first atomic bomb dropped on the Japanese city of Hiroshima on August 6, 1945 was codenamed as 'Little Boy' by the US, while the codename for the second bomb detonated over Nagasaki was 'Fat Man.' The codenames were reportedly based on design shapes of the bombs. The explosions remain the only known use of nuclear weapons in the world.