Little Boy was the codename of the atomic bomb dropped on Hiroshima during World War II. The United States used the uranium gun-type weapon on 6 August 1945.
The bomb was developed by the Manhattan Project, the secret Allied program that built the first nuclear weapons. Little Boy contained highly enriched uranium and used a gun-assembly design to bring separate pieces together rapidly. Unlike the plutonium implosion design used at Nagasaki, it was not tested in a full-scale nuclear explosion before combat use.
Little Boy is often confused with Fat Man, the different atomic bomb used against Nagasaki three days later. The Gadget was the name of the plutonium device detonated in the Trinity test in New Mexico, not the Hiroshima weapon. The B-29 Enola Gay carried Little Boy to Hiroshima.