Little Boy
What was Little Boy?
How was Little Boy designed to work?
What were the immediate effects of the Little Boy explosion?
Did the dropping of Little Boy lead to the end of World War II?
Little Boy, the first nuclear bomb ever used in warfare. It was dropped on the Japanese city of Hiroshima by the United States on August 6, 1945, during World War II. Little Boy was assembled at Tinian, an island in the Northern Mariana Islands, and delivered by the Boeing B-29 bomber Enola Gay. The explosion, which was equivalent to 15 kilotons of TNT, instantly killed an estimated 70,000 people and more than 100,000 people by the end of the year. Tens of thousands were to die in the following years because of the effects of radiation. Two-thirds of Hiroshima was destroyed. The detonation of Little Boy, followed by the bombing of Nagasaki using the Fat Man bomb, led to a Japanese surrender and the end of the war within a month.
Design and development
Little Boy was the world’s second nuclear explosion, after the Trinity test at the Alamogordo Bombing and Gunnery Range in New Mexico on July 16, 1945. Little Boy’s design involved firing one piece of uranium-235 into another to trigger an explosion.
Little Boy used a gun tube assembly method, originally planned for Fat Man but abandoned because of the high neutron emission rate of reactor-produced plutonium. The gun assembly ensured that the uranium-235 would achieve critical mass quickly, guaranteeing that the heat from the first reaction would not blow apart all the fuel before it was consumed. The main target case held a tungsten carbide tamper, a component which served a dual purpose as a neutron reflector to increase the number of critical masses in the uranium core and a highly dense tamper to hold the core together on impact to increase the explosive yield. The encased uranium core weighed 141 pounds (64 kg) when fully assembled. The overall bomb weighed 8,900 pounds (approximately 4,000 kg). Steel plates were bolted all around the assembly to form a compartment containing the electrical and mechanical components needed to detonate the bomb.
Little Boy’s design was highly volatile. Once the propellant for the bomb was loaded, any ignition could have caused a full-scale explosion. The gun tube assembly method had also never been tested before, as all of the purified uranium-235 produced to that date had been reserved for the bombing of Japan. No subsequent nuclear explosions used the Little Boy design, though five other Little Boy units had been built.
Delivery and deployment
On July 16, just hours after the successful completion of the Trinity test, the gun assembly mechanism for Little Boy, along with roughly half of the U.S. supply of uranium-235 and several Los Alamos technicians, was shipped to Tinian from San Francisco aboard the USS Indianapolis. The remainder of the U.S. uranium-235 stockpile was flown to Tinian on transport planes. Upon the arrival of the Indianapolis at Tinian on July 26, assembly began on the bomb.
Col. Paul W. Tibbets, Jr., the commander of the 509th Composite Group of the Twentieth Air Force, piloted the B-29 that would drop Little Boy. Shortly before taking off at approximately 2:45 am on August 6, 1945, Tibbets asked a maintenance worker to paint his mother’s name—Enola Gay—on the nose of the aircraft. Two other B-29s accompanied the Enola Gay to serve as observation and camera planes. Once the Enola Gay was airborne and at a safe distance from the Tinian base, weaponeer Capt. William Parsons added the final firing components to Little Boy.
The trusted destination for professionals, college students, and lifelong learners.
SUBSCRIBE
The Enola Gay ascended to an attack altitude of 31,000 feet (9,450 meters). Just after 8:00 am local time, the crew of the Enola Gay sighted Hiroshima. At around 8:12 am Tibbets relinquished control of the aircraft to his bombardier, Maj. Thomas Ferebee, who began his bombing run. Ferebee’s aim point was the Aioi Bridge over the Ōta River. Tibbets ordered his crew to don their protective goggles, and at 8:15 am the bomb was released.
It took roughly 45 seconds for Little Boy to descend to an altitude of 1,900 feet (580 meters), at which point it exploded in the sky directly above Shima Hospital. Within a fraction of a second of the detonation, the temperature at ground level exceeded 7,000 °C (12,600 °F), and a powerful blast wave scoured the landscape. Out of a population of 343,000 inhabitants, some 70,000 people were killed instantly, and by the end of the year the death toll had surpassed 100,000. Two-thirds of the city area was destroyed. A massive mushroom cloud rose to a height of more than 40,000 feet (more than 12 km). Although less than 2 percent of the uranium-235 contained in Little Boy had achieved fission, the bomb was horrifying in its destructive power. The explosive yield was the equivalent of 15 kilotons of TNT. A series of shockwaves rocked the Enola Gay as it departed the area, and at a distance of nearly 400 miles (640 km) the mushroom cloud was still visible.
Later that day, U.S. Pres. Harry S. Truman announced the news of the Hiroshima bombing to the American people.
Casualties and the effects of the bombing
The bomb destroyed 26 of the 33 modern firefighting stations in Hiroshima, killing or severely injuring three-fourths of the firefighting personnel. Of 298 registered physicians, only 30 escaped injury and were able to care for survivors. More than 1,800 of the city’s 2,400 nurses and orderlies were killed or seriously injured. Every hospital except one was destroyed or badly damaged. Electric power plants, railroads, telephones, and telegraph lines were all out of commission.
An estimated 60 percent of the deaths were caused by burns, including radiation burns. Mechanical injuries from flying debris and collapsing buildings are estimated to have caused another 30 percent of deaths, with the last 10 percent of deaths arising from radiation injuries, caused entirely by gamma rays and neutrons emitted at the instant of explosion. The surviving victims of the bombings (known in Japan as hibakusha) were promised free medical care for life by the Japanese government. Nihon Hidankyo, an organization dedicated to the elimination of nuclear weapons, was founded by several hibakusha in 1956; the group was awarded the Nobel Prize for Peace in 2024.
Two days after the bombing of Hiroshima, the Soviet Union declared war on Japan. On August 9 a second bomb, Fat Man, was dropped on Nagasaki. Japan surrendered on August 14; the signing of the formal papers of surrender on September 2, 1945, led to the end of World War II.
Extensive reconstruction began in both cities during the U.S. occupation of Japan. In Hiroshima a comprehensive planning scheme was enacted in 1950, and the city quickly became an industrial center for the region. Peace Memorial Park in Hiroshima is dedicated to those killed by the bomb, and the ruined shell of the Hiroshima Prefectural Industrial Promotion Hall (now known as the Atomic Bomb Dome) was designated a UNESCO World Heritage site in 1996.