Little Boy was the codename of the atomic bomb dropped on Hiroshima before Japan's surrender.
A U.S. B-29 bomber, Enola Gay, dropped Little Boy on August 6, 1945. The uranium-235 gun-type weapon detonated over Hiroshima, a major Japanese city on Honshu, causing immense destruction and loss of life.
Little Boy was the first nuclear weapon used in warfare. Its design differed from Fat Man, the plutonium implosion weapon dropped on Nagasaki three days later. The two bombs were developed under the Manhattan Project, the Allied program that produced nuclear weapons during World War II.
Japan did not surrender immediately after Hiroshima. The Soviet Union declared war on Japan on August 8, and Fat Man was dropped on Nagasaki on August 9. Japan then announced its decision to surrender, followed by the formal signing on September 2.