It contained 64 kg (141 lb) of highly enriched uranium, although less than a kilogram underwent nuclear fission.

Fission was accomplished by shooting a hollow cylinder (the 'bullet') onto a solid cylinder of the same material (the 'target') by means of a charge of nitrocellulose propellant powder. Like Thin Man, it was a gun-type fission weapon, but it derived its explosive power from the nuclear fission of uranium-235, whereas Thin Man was based on fission of plutonium-239. Little Boy was developed by Lieutenant Commander Francis Birch's group at the Manhattan Project's Los Alamos Laboratory during World War II, a reworking of their unsuccessful Thin Man nuclear bomb. The Hiroshima bombing was the second man-made nuclear explosion in history, after the Trinity nuclear test. It exploded with an energy of approximately 15 kilotons of TNT (63 TJ) and caused widespread death and destruction throughout the city. Tibbets, Jr., commander of the 509th Composite Group of the United States Army Air Forces and Captain Robert A. The bomb was dropped by the Boeing B-29 Superfortress Enola Gay piloted by Colonel Paul W. It was the first nuclear weapon used in warfare. ' Little Boy' was the codename for the type of atomic bomb dropped on the Japanese city of Hiroshima on 6 August 1945 during World War II.