Little Boy was the second nuclear bomb created by J. Robert Oppenheimer, Richard Feyman and pals during the Manhattan Project. The first bomb was a plutonium weapon tested at Alamogordo, N.M. on July 16, 1945.
Little Boy was a "gun type" uranium based nuclear bomb, enriched with U-235. It was developed under the Thin Man program (oppose Fat Man). The weapon used was designated L11. There were no others, they only had enough enriched uranium for one bomb.
Little Boy was dropped from the B-29 bomber Enola Gay on August 6. The bomb was highly unstable. Had the Enola Gay crashed en route, Little Boy would probably have detonated.
It exploded as planned at an altitude of 1,900 feet, having missed the drop point at Hiroshima's Aioi Bridge by less than 600 feet. 66,000 people died instantly, and the eventual death toll was 140,000.