ASSEMBLING AND DELIVERING THE URANIUM BOMB

(1945)
Processes > Bomb Testing and Weapon Effects

Both the uranium gun bomb—Little Boy—and the plutonium implosion bomb—Fat Man—were largely pre-assembled at Los Alamos. Final assembly took place at Tinian Island in the Marianas group in the western Pacific Ocean. With an operating B-29 airfield, Tinian was used as the advance base for carrying out the delivery. The Manhattan Engineer District built special bomb assembly buildings, loading pits, and other facilities at the base. A thirty-seven-member team of scientists and specialists from Los Alamos was responsible for last-stage experiments and tests and final assembly of the bomb.

The major parts of the uranium bomb consisting of the combat ballistic case and the uranium-235 projectile departed Los Alamos for Kirtland Army Air Field in Albuquerque on July 14, 1945, in a convoy of a closed black truck and seven cars with security agents. From Albuquerque, the bomb, packed in a large crate and a small metal cylinder, was flown in an Air Force plane to Hamilton Field near San Francisco where it was moved to the Hunter's Point naval shipyard. In the early morning of July 16, the bomb was placed aboard the cruiser Indianapolis, which set sail almost immediately for Tinian via Hawaii. The Indianapolis arrived at Tinian on July 26 and disembarked the bomb that day. (Four days later, the Indianapolis was sunk by a Japanese submarine with the loss of almost 900 lives.) The final parts of the uranium bomb, including the uranium-235 target insert, were flown by two otherwise empty C-54 cargo planes from Albuquerque to Tinian, via Hamilton Field and Hawaii, arriving on the night of July 28-29. On July 30, the target insert, projectile, and initiators were assembled in the bomb.

Little Boy was ready for use as early as August 1, but weather conditions the first four days of the month proved unsuitable. On the morning of August 5, meteorologists predicted that visual bombing would be possible over the target cities the following day, and the go ahead to fly the mission was given. Technical teams loaded the bomb in the Enola Gay B-29 aircraft and completed final tests. Final briefing took place at midnight. Hiroshima was the primary target, followed by Kokura Arsenal and then Nagasaki. On August 6 at 2:45 a.m. (Tinian time), the Enola Gay piloted by Colonel Paul Tibbets lifted off the Tinian runway with Little Boy in her bomb bay. William S. Parsons, head of the Los Alamos delivery program and bomb commander for the flight, began arming procedures by placing powder in the gun fifteen minutes after takeoff. At 7:30 a.m., Parsons inserted the red plugs that completed the arming circuit and made the bomb active. The aircraft climbed to 32,700 feet, and at 8:47 Parsons tested the electronic fuzes. At 9:09, Hiroshima was sighted, and at approximately 9:15 (8:15 Hiroshima time) the Enola Gay released the 9,700-pound uranium bomb over the city. Forty-three seconds later, a huge explosion lit the morning sky as Little Boy detonated 1,900 feet above Hiroshima. For more on the bombing, see The Atomic Bombing of Hiroshima.

Sources and notes for this page

The text for this page is original to the Department of Energy's Office of History and Heritage Resources. Major sources consulted include the following. Richard G. Hewlett and Oscar E. Anderson, Jr., The New World, 1939-1946: Volume I, A History of the United States Atomic Energy Commission (Washington: U.S. Atomic Energy Commission, 1972), 401 Lillian Hoddeson, et. al., Critical Assembly: A Technical History of Los Alamos during the Oppenheimer Years, 1943-1945 (Cambridge: Cambridge University Press, 1993), 265, 385, 390-92; Vincent C. Jones, Manhattan: The Army and the Atomic Bomb, United States Army in World War II (Washington: Center of Military History, United States Army, 1988), 536-38; and Leslie M. Groves, Now It Can Be Told: The Story of the Manhattan Project (New York: Harper, 1962), 305-8, 315-18. The photograph of "Little Boy" is courtesy the U.S. Army Corps of Engineers (via the National Archives). The photograph of the Enola Gay landing at Tinian Island is courtesy the USAF.