Little Boy was a type of atomic bomb created by the Manhattan Project during World War II. The name is also often used to describe the specific bomb (L-11) used in the bombing of the Japanese city of Hiroshima by the Boeing B-29 Superfortress Enola Gay on 6 August 1945, making it the first nuclear weapon used in warfare, and the second nuclear explosion in history, after the Trinity nuclear test. It exploded with an energy of approximately 15 kilotons of TNT (63 TJ) and had an explosion radius of approximately 1.3 kilometres (0.81 mi) which caused widespread death across the city. It was a gun-type fission weapon which used uranium that had been enriched in the isotope uranium-235 to power its explosive reaction.

Little Boy was developed by Lieutenant Commander Francis Birch's group at the Los Alamos Laboratory. It was the successor to a plutonium-fueled gun-type fission design, Thin Man, which was abandoned in 1944 after technical difficulties were discovered. Little Boy used a charge of cordite to fire a hollow cylinder (the "bullet") of highly enriched uranium through an artillery gun barrel into a solid cylinder (the "target") of the same material. The design was highly inefficient: the weapon used on Hiroshima contained 64 kilograms (141 lb) of uranium, but less than a kilogram underwent nuclear fission. Unlike the implosion design developed for the Trinity test and the Fat Man bomb design that was used against Nagasaki, which required sophisticated coordination of shaped explosive charges, the simpler but inefficient gun-type design was considered almost certain to work, and was never tested prior to its use at Hiroshima.

After the war, numerous components for additional Little Boy bombs were built. By 1950, at least five weapons were completed; all were retired by November 1950.

There are two primary accounts of how the first atomic bombs got their names. Los Alamos Laboratory and Project Alberta physicist Robert Serber stated, many decades after the fact, that he had named the first two atomic bomb designs during World War II based on their shapes: Thin Man and Fat Man. The "Thin Man" was a long, thin device, and its name came from the Dashiell Hammett detective novel and series of movies about The Thin Man. The "Fat Man" was round and fat so it was named after Kasper Gutman, a rotund character in Hammett's 1930 novel The Maltese Falcon, played by Sydney Greenstreet in the 1941 film version. Little Boy was named by others as an allusion to Thin Man since it was based on its design. It was also sometimes referred to as the "Mark I" nuclear bomb design, with "Mark II" referring to the abandoned Thin Man, and "Mark III" to the "Fat Man."

In September 1945, another Project Alberta physicist, Norman F. Ramsey, stated in his brief "History of Project A," that the early bomb ballistic test shapes designs were referred to as "Thin Man" and "Fat Man" by (unspecified) "Air Force representatives" for "security reasons," so that their communications over telephones sounded "as if they were modifying a plane to carry Roosevelt (the Thin Man) and Churchill (the Fat Man)," as opposed to modifying the B-29s to carry the two atomic bomb shapes as part of Project Silverplate in late 1943.

Another explanation of the names, from a classified United States Air Force history of Project Silverplate from the 1950s, implies a possible reconciliation of the two versions: that the terms "Thin Man" and "Fat Man" were first developed by someone at or from Los Alamos (i.e., Serber), but were consciously adopted by the officers in Silverplate when they were adopting their own codenames for their own project (including "Silverplate"). As Silverplate involved modifying B-29s for a secret purpose, deliberately using codenames that would align with modifying vehicles for Roosevelt and Churchill would serve their needs well.

Because of its perceived simplicity, the gun-type nuclear weapon design was the first approach pursued by the scientists working on bomb design during the Manhattan Project. In 1942, it was not yet known which of the two fissile materials pathways being simultaneously pursued—uranium-235 or plutonium-239—would be successful, or if there were significant differences between the two fuels that would impact the design work. Coordination with British scientists in May 1942 convinced the American scientists, led by J. Robert Oppenheimer, that the atomic bomb would not be difficult to design and that the difficulty would lie only in the production of fuel. Calculations in mid-1942 by theoretical physicists working on the project reinforced the idea that an ordinary artillery gun barrel would be able to impart sufficient velocity to the fissile material projectile.

Several different weapon designs, including autocatalytic assembly, a nascent version of implosion, and alternative gun designs (e.g., using high explosives as a propellent, or creating a "double gun" with two projectiles) were pursued in the early years of the project, while the facilities to manufacture fissile material were being constructed. The belief that the gun design would be an easy engineering task once fuel was available led to a sense of optimism at Los Alamos, although Oppenheimer established a small research group to study implosion as a fallback in early 1943. A full ordnance program for gun-design development was established by March 1943, with expertise provided by E.L. Rose, an experienced gun designer and engineer. Work was begun to study the properties of barrels, internal and external ballistics, and tampers of gun weapons. Oppenheimer led aspects of the effort, telling Rose that "at the present time [May 1945] our estimates are so ill founded that I think it better for me to take responsibility for putting them forward." He soon delegated the work to Naval Captain William Sterling Parsons, who, along with Ed McMillan, Charles Critchfield, and Joseph Hirschfelder would be responsible for rendering the theory into practice.