Delivering the Bomb

From the Assembly to the Atomic Strikes on Japan

In the final weeks of World War II, a tightly coordinated mix of logistics, secrecy, and scientific focus led to the delivery of the atomic bombs. The mission, shrouded in silence and guarded at every step, unfolded as a series of high-stakes operations across oceans and continents. From Los Alamos to the B-29 bombers on Tinian Island, and during days awaiting presidential orders, the Manhattan Project men stood on the edge of history-defining change.

The Journey Begins: Los Alamos to San Francisco

At Los Alamos, New Mexico, years of secret scientific effort led to the assembly of two bombs: “Little Boy,” a uranium-based gun-type device, and “Fat Man,” a plutonium implosion device.¹

Little Boy’s gun-type design inspired confidence, requiring no full-scale test. Fat Man’s implosion design was successfully proven during the July 16, 1945, Trinity test.²

On July 14, 1945, the uranium projectile and target disks for Little Boy were loaded onto the cruiser USS Indianapolis, which departed from the Hunters Point Naval Shipyard in San Francisco under utmost secrecy.³

The Tragedy of the Indianapolis

On July 26, 1945, the Indianapolis arrived at Tinian Island in the Northern Marianas, part of the vast U.S. air base system. The bomb components were immediately offloaded and handed over to the Army Air Forces' 509th Composite Group, a special unit created for the atomic bomb mission under the command of Colonel Paul Tibbets.⁴

After delivering the bomb parts, the Indianapolis continued its journey, only to be torpedoed by a Japanese submarine days later. Nearly 900 men died, many of them victims of shark attacks while stranded at sea—an enduring tragedy tied to the bomb’s delivery.⁵

Assembly on Tinian

Once on Tinian, bomb components were sent to the Project Alberta assembly teams, comprising physicists, technicians, and military personnel under Navy Captain William “Deak” Parsons.⁶ The uranium core and related parts of Little Boy were reassembled in a secure area, with final arming postponed until the plane was airborne.

Meanwhile, Fat Man's plutonium core and high-explosive lenses were flown in separately from Kirtland Army Air Field in New Mexico.⁷ The bomb casings had arrived earlier via specially modified B-29 flights.

Waiting for the Order: The Days Before Hiroshima

From July 31 to August 5, 1945, the crews of the 509th waited in tense anticipation. The weather over Japan was a key factor—clouds or storms could obstruct the visual aiming necessary for the Norden bombsight (a device that let crews visually aim bombs with great accuracy).⁸ Colonel Tibbets had selected five potential targets: Hiroshima, Kokura, Niigata, Nagasaki, and Kyoto. (Kyoto was removed after personal intervention by Secretary of War Henry Stimson.)⁹

On August 2, President Truman, in Potsdam, received word that all bomb elements were in place. Final approval came on August 4.¹⁰ Good weather on August 6 favored Hiroshima. Colonel Tibbets would lead the mission, piloting the Enola Gay.

August 6, 1945: Hiroshima

At 2:45 AM, August 6, the Enola Gay lifted off from Tinian, carrying Little Boy and a hand-picked crew. At 8:15 AM Hiroshima time, the bomb was dropped from 31,000 feet. It detonated approximately 1,900 feet above the city center.¹¹ Between 70,000 and 80,000 people died instantly, with tens of thousands more succumbing in the weeks and months that followed.¹²

Despite the bomb’s horrific destruction, Japan did not immediately surrender. Tokyo’s leaders were stunned, but some believed the U.S. had only one bomb.

The Days Between Strikes

Back on Tinian, the 509th Composite Group continued preparations. Fat Man was already being assembled. On August 7, test flights and camera runs were conducted.¹³ Final assembly took place in one of the Tinian bomb pits (concrete enclosures used for bomb storage and assembly) under heavy guard. Unlike Little Boy, Fat Man required greater technical precision due to its implosion mechanism and plutonium core.

President Truman had authorized further strikes if Japan refused to surrender. The next mission was initially scheduled for August 11, but bad weather forecasts led to the rescheduling for August 9.¹⁴

August 9, 1945: Nagasaki

In the early hours of August 9, the B-29 Bockscar, piloted by Major Charles Sweeney, took off carrying Fat Man. The primary target, Kokura, was obscured by smoke and cloud cover, forcing the crew to divert to the secondary target, Nagasaki. At 11:02 AM, Fat Man detonated above the Urakami district.¹⁵

Although the terrain limited the bomb’s destructive radius, the device still killed about 40,000 people instantly.¹⁶

Aftermath and Surrender

The combined effect of the atomic bombings and the Soviet invasion of Manchuria on August 9 pushed Japan toward surrender. On August 15, Emperor Hirohito delivered a historic radio address accepting the terms of the Potsdam Declaration.¹⁷

Conclusion

The delivery of the atomic bombs involved a complex web of scientific engineering, military coordination, and presidential authorization. Each day between the arrival of the components and their ultimate deployment was filled with tension and uncertainty. The men who carried out these missions lived with the knowledge that they were unleashing a force—nuclear fission, which splits atoms and releases immense energy—that would not only end a war but also redefine the future of global conflict.

Footnotes

1. Richard Rhodes, The Making of the Atomic Bomb (New York: Simon & Schuster, 1986), 589–590.

2. Ibid., 672–675.

3. William S. Parsons and Warner K. Schmidt, "The Flight of the Enola Gay," U.S. Naval Institute Proceedings (August 1960): 36–45.

4. Paul W. Tibbets, The Tibbets Story (New York: Stein and Day, 1978), 192–198.

5. Lynn Vincent and Sara Vladic, Indianapolis: The True Story of the Worst Sea Disaster in U.S. Naval History (New York: Simon & Schuster, 2018), 234–252.

6. James Kunetka, City of Fire: Los Alamos and the Birth of the Atomic Age, 1943–1945 (Englewood Cliffs, NJ: Prentice Hall, 1978), 292–294.

7. Michael D. Gordin, Five Days in August: How World War II Became a Nuclear War (Princeton: Princeton University Press, 2007), 67–69.

8. Tibbets, The Tibbets Story, 204–206.

9. Gar Alperovitz, The Decision to Use the Atomic Bomb (New York: Knopf, 1995), 80.

10. Rhodes, The Making of the Atomic Bomb, 728.

11. Tibbets, The Tibbets Story, 212–213.

12. John Hersey, Hiroshima (New York: Alfred A. Knopf, 1946), 6–10.

13. Gordin, Five Days in August, 79–82.

14. Barton J. Bernstein, “The Atomic Bombings Reconsidered,” Foreign Affairs 74, no. 1 (1995): 135–152.

15. Charles Sweeney, War’s End: An Eyewitness Account of America's Last Atomic Mission (New York: Avon, 1997), 156–163.

16. Ibid., 165.

17. Herbert P. Bix, Hirohito and the Making of Modern Japan (New York: HarperCollins, 2000), 518–522.