Little Boy Aug 6 1945 " Little Boy " was the codename for the atomic bomb dropped on the Japanese city of Hiroshima on 6 August 1945 during World War II by the Boeing B-29 Superfortress Enola Gay , piloted by Colonel Paul W. Tibbets, Jr. , commander of the 509th Composite Group of the United States Army Air Forces . It was the first atomic bomb to be used in warfare . The Hiroshima bombing was the second artificial nuclear explosion in history, after the Trinity test , and the first uranium -based detonation. It exploded with an energy of approximately 15 kilotons of TNT (63 TJ). The bomb caused significant destruction to the city of Hiroshima and its occupants. Fat Man Aug 9 1945 " Fat Man " was the codename for the atomic bomb that was detonated over the Japanese city of Nagasaki by the United States on 9 August 1945. It was the second of the only two nuclear weapons ever used in warfare , the first being Little Boy , and it