"Fat Man" (also known as Mark III) was the type of nuclear weapon the United States detonated over the Japanese city of Nagasaki on 9 August 1945.

The first one was built by scientists and engineers at Los Alamos Laboratory using plutonium manufactured at the Hanford Site and was dropped from the Boeing B-29 Superfortress Bockscar piloted by Major Charles Sweeney.

The first of that type to be detonated was the Gadget in the Trinity nuclear test less than a month earlier on 16 July at the Alamogordo Bombing and Gunnery Range in New Mexico.

Robert Oppenheimer held conferences in Chicago in June 1942, and in Berkeley, California, in July, at which various engineers and physicists discussed nuclear bomb design issues.

[4] Conant consulted Ernest Lawrence and Arthur Compton, who acknowledged that their scientists at Berkeley and Chicago, respectively, knew about the problem, but they could offer no ready solution.

Conant informed Manhattan Project director Brigadier General Leslie R. Groves Jr., who in turn assembled a special committee consisting of Lawrence, Compton, Oppenheimer, and McMillan to examine the issue.

[5] Oppenheimer reviewed his options in early 1943 and gave priority to the gun-type weapon,[3] but he created the E-5 Group at the Los Alamos Laboratory under Seth Neddermeyer to investigate implosion as a hedge against the threat of pre-detonation.

[9] Neddermeyer discarded Serber and Tolman's initial concept of implosion as assembling a series of pieces in favor of one in which a hollow sphere was imploded by an explosive shell.

After reviewing Neddermeyer's studies, and discussing the matter with Edward Teller, von Neumann suggested the use of high explosives in shaped charges to implode a sphere, which he showed could not only result in a faster assembly of fissile material than was possible with the gun method, but greatly reduce the amount of material required because of the resulting higher density.

[12] The prospect of more-efficient nuclear weapons impressed Oppenheimer, Teller, and Hans Bethe, but they decided that an expert on explosives would be required.

"[13] The distance required to accelerate the plutonium to speeds where pre--detonation would be less likely would need a gun barrel too long for any existing or planned bomber.

All gun-type work in the Manhattan Project was re-directed towards the Little Boy, enriched uranium gun design, and the Los Alamos Laboratory was reorganized with almost all of the research focused on the problems of implosion for the Fat Man bomb.

The only Allied bombers considered capable of carrying the Fat Man without major modification were the British Avro Lancaster and the American Boeing B-29 Superfortress.

[25] At the time, the B-29 represented the epitome of bomber technology with significant advantages in maximum takeoff weight, range, speed, flight ceiling, and survivability.

[22] In drop tests in early weeks, the Fat Man missed its target by an average of 1,857 feet (566 m), but this was halved by June as the bombardiers became more proficient with it.

[37] The explosion symmetrically compressed the plutonium to twice its normal density before the "Urchin" added free neutrons to initiate a fission chain reaction.

[43] The first plutonium core was transported with its polonium-beryllium modulated neutron initiator in the custody of Project Alberta courier Raemer Schreiber in a magnesium field carrying case designed for the purpose by Philip Morrison.

Three Fat Man high-explosive pre-assemblies (designated F31, F32, and F33) were picked up at Kirtland on 28 July by three B-29s: Luke the Spook and Laggin' Dragon from the 509th Composite Group's 393d Bombardment Squadron, and another from the 216th Army Air Forces Base Unit.

It was placed inside its ellipsoidal aerodynamic bombshell, which was painted mustard yellow, and wheeled out, where it was signed by nearly 60 people, including Purnell, Brigadier General Thomas F. Farrell, and Parsons.

[52][53][54] It was then wheeled to the bomb bay of the B-29 Superfortress named Bockscar after the plane's command pilot Captain Frederick C. Bock,[55] who flew The Great Artiste with his crew on the mission.

Bockscar was flown by Major Charles W. Sweeney and his crew, with Commander Frederick L. Ashworth from Project Alberta as the weaponeer in charge of the bomb.

[58] Kokura was obscured by clouds and drifting smoke from fires started by a major firebombing raid by 224 B-29s on nearby Yahata the previous day.

Three bomb runs were made over the next 50 minutes, burning fuel and repeatedly exposing the aircraft to the heavy defenses of Yahata, but the bombardier was unable to drop visually.

[57] There was poor visibility due to cloud cover, and the bomb missed its intended detonation point by almost two miles, so the damage was somewhat less extensive than that in Hiroshima.

A total of 60,000–80,000 fatalities resulted, including from long-term health effects, the strongest of which was leukemia with an attributable risk of 46% for bomb victims.

The second bomb was nicknamed Helen of Bikini and was placed without its tail fin assembly in a steel caisson made from a submarine's conning tower; it was detonated 90 feet (27 m) beneath the landing craft USS LSM-60.

The North American B-45 Tornado, Convair XB-46, Martin XB-48, and Boeing B-47 Stratojet bombers had bomb bays sized to carry the Grand Slam, which was much longer but not as wide as the Fat Man.

In November 1945, the Army Air Forces asked Los Alamos for 200 Fat Man bombs, but there were only two sets of plutonium cores and high-explosive assemblies at the time.

The Army Air Forces wanted improvements to the design to make it easier to manufacture, assemble, handle, transport, and stockpile.

It was replaced by improved versions known as Mods 1 and 2 which contained a number of minor changes, the most important of which was that they did not charge the X-Unit firing system's capacitors until released from the aircraft.