The uranium atomic bomb dropped on Hiroshima was code-named Little Boy.
The United States dropped Little Boy from the B-29 bomber Enola Gay on August 6, 1945. The bomb used uranium-235 in a gun-type design, in which one subcritical piece of uranium was fired into another to create a supercritical chain reaction. It was the first nuclear weapon used in warfare.
The explosion devastated Hiroshima and caused enormous immediate and long-term loss of life. Three days later, the United States dropped a different atomic bomb, Fat Man, on Nagasaki. Fat Man used plutonium and an implosion design, so the two weapons differed in both material and engineering.
The code name Little Boy is sometimes confused with Thin Man, an earlier plutonium-gun design that was abandoned because the plutonium produced by the Hanford reactor was unsuitable for that mechanism. Japan announced its acceptance of surrender terms on August 15, after the Hiroshima and Nagasaki bombings and the Soviet entry into the war against Japan.