“Little Boy” was the codename for the type of atomic bomb utilizing uranium 235. It was dropped on Hiroshima on August 6, 1945 and was the first atomic bomb to be used as a weapon. The Hiroshima bombing was the second artificial nuclear explosion in history, after the Trinity test, and the first uranium-based detonation. It exploded with the energy of 15 kilotons of TNT.
“Fat Man" was the codename for the type of atomic bomb that utilized plutonium as the active radioactive element. It was detonated over Nagasaki on August 9, 1945 and was the second of two nuclear weapons to be used in warfare. It exploded with the energy of 21 kilotons of TNT.
The names for all three atomic bomb design projects during World War II, Fat Man, Thin Man (never used), and Little Boy, were created by Robert Serber, a former student of Los Alamos Laboratory director Robert Oppenheimer who worked on the Manhattan Project. According to Serber, he chose them based on their design shapes. The "Thin Man" was a long device, and the name came from the Dashiell Hammett detective novel and series of movies by the same name; the "Fat Man" was round and fat, and was named after Sydney Greenstreet's "Kasper Gutman" character in The Maltese Falcon. Little Boy would come last and was named after Elisha Cook, Jr.'s character in the same film, as referred to by Humphrey Bogart.