World War II: "Little Boy" Atomic Bomb

Little Boy atomic bomb
LB (Little Boy) unit on trailer cradle in pit. [Note bomb bay door in upper right-hand corner.] , 08/1945.

National Archives & Records Administration

Little Boy was the first atomic bomb used against Japan in World War II and was detonated over Hiroshima on August 6, 1945. The design was the work of a team led by Lieutenant Commander Francis Birch at the Los Alamos Laboratory. A gun-type fission weapon, the Little Boy design utilized uranium-235 to create its nuclear reaction. Delivered to Tinian in the Marianas, the first Little Boy was carried to its target by the B-29 Superfortresses Enola Gay flown by Colonel Paul W. Tibbets, Jr. of the 509th Composite Group. The Little Boy design was briefly retained in the years after World War II but was quickly eclipsed by newer weapons.

The Manhattan Project

Overseen by Major General Leslie Groves and scientist Robert Oppenheimer, the Manhattan Project was the name given to the United States' efforts to build nuclear weapons during World War II. The first approach pursued by the project was the use of enriched uranium to create a weapon, as this material was known to be fissionable. To meet the project's needs, enriched uranium production began at a new facility in Oak Ridge, TN in early 1943. Around the same time, scientists began experimenting with various bomb prototypes at the Los Alamos Design Laboratory in New Mexico.

Uranium Designs

Early work focused on "gun-type" designs which fired one piece of uranium into another to create a nuclear chain reaction. While this approach proved promising for uranium-based bombs, it was less so for those utilizing plutonium. As a result, the scientists at Los Alamos began developing an implosion design for a plutonium-based bomb as this material was relatively more plentiful. By July 1944, the bulk of the research was focused on the plutonium designs and the uranium gun-type bomb was less of a priority.

Leading the design team for the gun-type weapon, Lieutenant Commander Francis Birch succeeded in convincing his superiors that the design was worth pursuing if only as a back-up in case the plutonium bomb design failed. Pushing forward, Birch's team produced specifications for the bomb design in February 1945. Moving into production, the weapon, minus its uranium payload, was completed in early May. Dubbed the Mark I (Model 1850) and code-named "Little Boy," the bomb's uranium was not available until July. The final design measured 10 feet long and 28 inches in diameter.

Little Boy Design

A gun-type nuclear weapon, Little Boy relied on one mass of uranium-235 hitting another to create a nuclear reaction. As a result, the core component of the bomb was a smoothbore gun barrel through which the uranium projectile would be fired. The final design specified the use of 64 kilograms of uranium-235. Approximately 60% of this was formed into the projectile, which was a cylinder with a four-inch hole through the middle. The remaining 40% comprised the target which was a solid spike measuring seven inches long with a diameter of four inches.

Assembling Little Boy atomic bomb
Commander A. Francis Birch (left) assembles the bomb while physicist Norman Ramsey watches. Public Domain

When detonated, the projectile would be propelled down the barrel by a tungsten carbide and steel plug and would create a super-critical mass of uranium at impact. This mass was to be contained by a tungsten carbide and steel tamper and neutron reflector. Due to a lack of uranium-235, no full-scale test of the design occurred prior to the bomb's construction. Also, due to its relatively simplistic design, Birch's team felt that only smaller-scale, laboratory tests were necessary to prove the concept.

Though a design that virtually ensured success, Little Boy was relatively unsafe by modern standards, as several scenarios, such as a crash or electrical short circuit, could lead to a "fizzle" or accidental detonation. For detonation, Little Boy employed a three-stage fuse system which ensured that the bomber could escape and that it would explode at a preset altitude. This system employed a timer, barometric stage, and a set of doubly-redundant radar altimeters.

"Little Boy" Atomic Bomb

  • Type: Nuclear weapon
  • Nation: United States
  • Designer: Los Alamos Labratory
  • Length: 10 feet
  • Weight: 9,700 pounds
  • Diameter: 28 inches
  • Filling: Uranium-235
  • Yield: 15 kilotons of TNT

Delivery & Use

On July 14, several completed bomb units and the uranium projectile were shipped by train from Los Alamos to San Francisco. Here they were embarked aboard the cruiser USS Indianapolis. Steaming at high speed, the cruiser delivered the bomb components to Tinian on July 26. That same day, the uranium target was flown to the island in three C-54 Skymasters from the 509th Composite Group. With all of the pieces on hand, bomb unit L11 was chosen and Little Boy assembled.

Due to the danger of handling the bomb, the weaponeer assigned to it, Captain William S. Parsons, made the decision to delay inserting the cordite bags into the gun mechanism until the bomb was airborne. With the decision to use the weapon against the Japanese, Hiroshima was selected as the target and Little Boy was loaded aboard the B-29 Superfortress Enola Gay. Commanded by Colonel Paul Tibbets, Enola Gay took off on August 6 and rendezvoused with two additional B-29s, which had been loaded with instrumentation and photographic equipment, over Iwo Jima.

Enola Gay
Boeing B-29 Superfortress "Enola Gay" landing after the atomic bombing mission on Hiroshima, Japan, 1945. U.S. Air Force

Proceeding to Hiroshima, Enola Gay released Little Boy over the city at 8:15 AM. Falling for fifty-seven seconds, it detonated at the predetermined height of 1,900 feet with a blast equivalent to about 13-15 kilotons of TNT. Creating an area of complete devastation approximately two miles in diameter, the bomb, with its resulting shock wave and firestorm, effectively destroyed around 4.7 square miles of the city, killing 70,000-80,000 and injuring another 70,000. The first nuclear weapon used in wartime, it was quickly followed three days later by the use of "Fat Man," a plutonium bomb, on Nagasaki.

Postwar

As it was not expected that the Little Boy design would be used again, many of the plans for the weapon were destroyed. This caused a problem in 1946 when a shortage of plutonium for new weapons led to the need to construct several uranium-based bombs as a stopgap. This resulted in a successful effort to recreate the original design and produced six assemblies. In 1947, the U.S. Navy Bureau of Ordnance built 25 Little Boy assemblies though by the following year there was only enough fissionable material to arm ten. The last of the Little Boy units were removed from the inventory in January 1951.