What was the codename for the atomic bomb dropped on Hiroshima?

The codename for the atomic bomb dropped on Hiroshima was Little Boy. The uranium-235 bomb was carried by the B-29 bomber Enola Gay and detonated over Hiroshima on 6 August 1945.

Little Boy was developed at Los Alamos as part of the Manhattan Project. It used a gun-type design: conventional explosives fired one piece of highly enriched uranium into another, creating a supercritical mass and a nuclear fission explosion. Because engineers considered the mechanism reliable, the weapon was not tested before combat use.

The bomb contained about 64 kilograms of enriched uranium, though less than one kilogram actually underwent fission. Its yield was approximately 15 kilotons of TNT. The explosion became the first use of a nuclear weapon in warfare and killed or fatally injured large numbers of people in Hiroshima, with many more dying later from burns, injuries, and radiation effects.

Trinity was the code name for the first nuclear test in New Mexico. Fat Man was the plutonium bomb dropped on Nagasaki on 9 August 1945, while Thin Man was an abandoned earlier design.