Little Boy was the codename of the atomic bomb dropped on Hiroshima before Japan surrendered in 1945.
The United States dropped Little Boy from the B-29 bomber Enola Gay on August 6, 1945. The uranium-235 weapon detonated over Hiroshima at approximately 8:15 a.m. local time. The bombing destroyed much of the city and caused enormous immediate and long-term loss of life.
Little Boy used a gun-type fission design, in which one subcritical piece of uranium was fired into another to create a supercritical mass. It was simpler than the implosion design used for the second wartime atomic bomb.
The Hiroshima attack did not produce an immediate public Japanese surrender. The Soviet Union entered the war against Japan on August 9, and the United States dropped a second atomic bomb on Nagasaki that same day. Japan then notified the Allies of its intention to surrender on August 10, subject to the Emperor’s position.
Little Boy is often confused with Fat Man, the plutonium implosion bomb used against Nagasaki. The Trinity test device, called The Gadget, was neither of the bombs used against Japanese cities.