"A 'Fat Man' bomb was dropped over Nagasaki, Japan, on Aug. 9, 1945, near the end of World War II. Released by the B-29 Bockscar, the 10,000-pound weapon was detonated at an altitude of approximately 1,800 feet over the city. The bomb had an explosive force (yield) of about 20,000 tons of TNT, about the same as the bomb dropped on Hiroshima. Because of Nagasaki's hilly terrain, however, the damage was somewhat less extensive than of the relatively flat Hiroshima.

'Fat Man' was an implosion-type weapon using plutonium. A subcritical sphere of plutonium was placed in the center of a hollow sphere of high explosive (HE). Numerous detonators located on the surface of the HE were fired simultaneously to produce a powerful inward pressure on the capsule, squeezing it and increasing its density. This resulted in a supercritical condition and a nuclear explosion.
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"The Mk I bomb, nicknamed "Little Boy," was the first nuclear weapon used in warfare. It was delivered by the B-29 Enola Gay (on display at the Smithsonian National Air & Space Museum), it detonated at an altitude of 1,800 feet over Hiroshima, Japan, on Aug. 6, 1945. The result of the Manhattan Project, begun in June 1942, "Little Boy" was a gun-type weapon, which detonated by firing one mass of uranium down a cylinder into another mass to create a self-sustaining nuclear reaction. Weighing about 9,000 pounds, it produced an explosive force equal to 20,000 tons of TNT.

"When constructed in 1945, the "Little Boy" on display was an operational weapon, but it has been completely demilitarized for display purposes. In 2004 the Department of Energy repaired and repainted the artifact at its Sandia National Laboratories in Albuquerque, N.M."