<Tr> <Th> Filling weight </Th> <Td> 140 lb (64 kg) </Td> </Tr> <Tr> <Th> Blast yield </Th> <Td> 15 kilotons of TNT (63 TJ; 0.7 g mass equivalent) </Td> </Tr> <P> "Little Boy" was the codename for the atomic bomb dropped on the Japanese city of Hiroshima on 6 August 1945 during World War II by the Boeing B - 29 Superfortress Enola Gay, piloted by Colonel Paul W. Tibbets, Jr., commander of the 509th Composite Group of the United States Army Air Forces . It was the first atomic bomb to be used in warfare . The Hiroshima bombing was the second artificial nuclear explosion in history, after the Trinity test, and the first uranium - based detonation . It exploded with an energy of approximately 15 kilotons of TNT (63 TJ). The bomb caused significant destruction to the city of Hiroshima and its occupants . </P> <P> Little Boy was developed by Lieutenant Commander Francis Birch's group of Captain William S. Parsons's Ordnance (O) Division at the Manhattan Project's Los Alamos Laboratory during World War II . Parsons flew on the Hiroshima mission as weaponeer . The Little Boy was a development of the unsuccessful Thin Man nuclear bomb . Like Thin Man, it was a gun - type fission weapon, but derived its explosive power from the nuclear fission of uranium - 235, whereas Thin Man was based on fission of plutonium - 239 . Fission was accomplished by shooting a hollow cylinder of enriched uranium (the "bullet") onto a solid cylinder of the same material (the "target") by means of a charge of nitrocellulose propellant powder . It contained 64 kg (141 lb) of enriched uranium, of which less than a kilogram underwent nuclear fission . Its components were fabricated at three different plants so that no one would have a copy of the complete design . </P>

What was the first atomic bomb made out of