<P> The design specifications were completed in February 1945, and contracts were let to build the components . Three different plants were used so that no one would have a copy of the complete design . The gun and breech were made by the Naval Gun Factory in Washington, D.C.; the target case and some other components were by the Naval Ordnance Plant in Center Line, Michigan; and the tail fairing and mounting brackets by the Expert Tool and Die Company in Detroit, Michigan . The bomb, except for the uranium payload, was ready at the beginning of May 1945 . The uranium - 235 projectile was completed on 15 June, and the target on 24 July . The target and bomb pre-assemblies (partly assembled bombs without the fissile components) left Hunters Point Naval Shipyard, California, on 16 July aboard the heavy cruiser USS Indianapolis, arriving on 26 July . The target inserts followed by air on 30 July . </P> <P> Although all of its components had been tested, no full test of a gun - type nuclear weapon occurred before the Little Boy was dropped over Hiroshima . The only test explosion of a nuclear weapon concept had been of an implosion - type device employing plutonium as its fissile material, and took place on 16 July 1945 at the Trinity nuclear test . There were several reasons for not testing a Little Boy type of device . Primarily, there was little uranium - 235 as compared with the relatively large amount of plutonium which, it was expected, could be produced by the Hanford Site reactors . Additionally, the weapon design was simple enough that it was only deemed necessary to do laboratory tests with the gun - type assembly . Unlike the implosion design, which required sophisticated coordination of shaped explosive charges, the gun - type design was considered almost certain to work . </P> <P> The danger of accidental detonation made safety a concern . Little Boy incorporated basic safety mechanisms, but an accidental detonation could still occur . Tests were conducted to see whether a crash could drive the hollow "bullet" onto the "target" cylinder resulting in a massive release of radiation, or possibly nuclear detonation . These showed that this required an impact of 500 times the force of gravity, which made it highly unlikely . There was still concern that a crash and a fire could trigger the explosives . If immersed in water, the uranium halves were subject to a neutron moderator effect . While this would not have caused an explosion, it could have created widespread radioactive contamination . For this reason, pilots were advised to crash on land rather than at sea . </P> <P> The Little Boy was 120 inches (300 cm) in length, 28 inches (71 cm) in diameter and weighed approximately 9,700 pounds (4,400 kg). The design used the gun method to explosively force a hollow sub-critical mass of uranium - 235 and a solid target cylinder together into a super-critical mass, initiating a nuclear chain reaction . This was accomplished by shooting one piece of the uranium onto the other by means of four cylindrical silk bags of cordite . The bomb contained 64 kg (141 lb) of enriched uranium . Most was enriched to 89% but some was only 50% uranium - 235, for an average enrichment of 80% . Less than a kilogram of uranium underwent nuclear fission, and of this mass only 0.6 g (0.021 oz) was transformed into several forms of energy, mostly kinetic energy, but also heat and radiation . </P>

Where did the atomic bomb detonated in hiroshima