The Hiroshima bomb was never tested, because its designers were sure it would work
Little Boy, the uranium bomb dropped on Hiroshima on 6 August 1945, was a crude design: an artillery barrel that fired one slug of enriched uranium into another. It carried 64 kilograms of uranium, yet under one kilogram actually split. Engineers trusted it so completely that no full-scale test preceded its use.
Early in the Manhattan Project, the gun method looked like the easy route. Talks with British scientists in May 1942 persuaded J. Robert Oppenheimer's team that designing a bomb would be simple and that making the fuel was the real obstacle. A gun designer, E. L. Rose, made a point that seemed obvious once said: the barrel only had to survive one shot, so it could be made far lighter than a normal cannon.
The first plan was a plutonium gun some 17 feet long, nicknamed Thin Man. In early 1944 Emilio Segrè's group analysed the first reactor-made plutonium and found the isotope plutonium-240 made it fission spontaneously too often. A plutonium gun would blow itself apart before a full explosion could develop. From July 1944 Los Alamos pivoted to the implosion design later used at Trinity and Nagasaki, while a smaller team kept the gun concept alive for uranium only, which allowed a much shorter weapon that fitted a B-29 bomb bay.
The names have competing origins. Physicist Robert Serber later said he named Thin Man and Fat Man after characters in Dashiell Hammett's stories, and Little Boy was a play on Thin Man. Another account says air force officers used the names so phone calls sounded like planes being fitted out for Roosevelt and Churchill. To guard the design, components were split among three plants, and parts crossed the Pacific aboard the cruiser USS Indianapolis in July 1945.
Skipping a test was partly a matter of supply. The K-25 plant produced about 30 kilograms of enriched uranium a month, and each bomb needed over 60, so a trial would have meant long delays. Dummy projectiles fired down the barrel confirmed the parts worked. The bomb released roughly 15 kilotons of TNT equivalent, a yield now used as a yardstick for earthquakes and eruptions. At least five more were built after the war, and all were retired by November 1950.
Source: Little Boy
