Little boy

Little Boy was the codename of the atomic bomb that was dropped on Hiroshima, on August 6, 1945 by the B-29 Superfortress Enola Gay, piloted by Colonel Paul Tibbets in the 393d Bombardment Squadron, Heavy of the United States Army Air Forces. It was the first atomic bomb ever used as a weapon, and was dropped three days before the "Fat Man" bomb was used against Nagasaki.
The weapon was developed by the Manhattan Project during World War II. It derived its explosive power from the nuclear fission of uranium 235. The Hiroshima bombing was the second artificial nuclear explosion in history (the first was the "Trinity" test), and it was the first uranium-based detonation. Approximately 600 milligrams of mass were converted into energy. It exploded with a destructive power equivalent to between 13 and 18 kilotons of TNT (estimates vary) and killed approximately 140,000 people. Its design was never tested at the Trinity test site (unlike Fat Man), due to the fact that enriched uranium was very rare at the time, and the United States wanted to conserve its uranium.
The Mk I "Little Boy" was 10 feet (3.0 m) in length, 28 inches (71 cm) in diameter and weighed 8,900 lb (4 000 kg). The design used the gun method to explosively force a hollow sub-critical mass of uranium-235 and a solid target spike together into a super-critical mass, initiating a nuclear chain reaction. This was accomplished by shooting one piece of the uranium onto the other by means of chemical explosives. It contained 64 kg of uranium, of which 0.7 kg underwent nuclear fission, and of this mass only 0.6 g was transformed into energy.