What was the codename of the atomic bomb used against Hiroshima?

Little Boy was the codename of the atomic bomb used against Hiroshima.

The United States dropped Little Boy on Hiroshima on August 6, 1945. It was a uranium-235 gun-type nuclear weapon and was carried by the B-29 Superfortress Enola Gay, commanded by Colonel Paul Tibbets.

Little Boy differed substantially from Fat Man, the plutonium implosion weapon later used against Nagasaki. Its design fired one subcritical piece of uranium into another to create the conditions for a nuclear chain reaction. Because the uranium design was considered sufficiently reliable, it was not tested before combat use.

The Hiroshima bombing was the first use of a nuclear weapon in warfare. The blast and its immediate effects devastated much of the city, while radiation injuries and other consequences continued afterward. Japan’s government did not announce surrender immediately; the Soviet Union declared war on Japan on August 8, and the Nagasaki bombing followed on August 9.