Atomic bombings of Hiroshima and Nagasaki

The release at 08:15 (Hiroshima time) went as planned, and the gravity bomb known as "Little Boy", a gun-type fission weapon with 60 kilograms (130 lb) of uranium-235, took 57 seconds to fall from the aircraft to the predetermined detonation height about 600 meters (2,000 ft) above the city. Due to crosswind, it missed the aiming point, the Aioi Bridge, by almost 800 feet (240 m) and detonated directly over Shima Surgical Clinic.[24] It created a blast equivalent to about 13 kilotons of TNT (54 TJ). (The U-235 weapon was considered very inefficient, with only 1.38% of its material fissioning.)[25] The radius of total destruction was about one mile (1.6 km), with resulting fires across 4.4 square miles (11 km2).[26] Americans estimated that 4.7 square miles (12 km2) of the city were destroyed. Japanese officials determined that 69% of Hiroshima's buildings were destroyed and another 6–7% damaged.[5]

70,000–80,000 people, or some 30%[27] of the population of Hiroshima were killed immediately, and another 70,000 injured.[28] Over 90% of the doctors and 93% of the nurses in Hiroshima were killed or injured—most had been in the downtown area which received the greatest damage.[29]

Although the U.S. had previously dropped leaflets warning civilians of air raids on twelve other Japanese cities,[30] the residents of Hiroshima were given no notice of the atomic bomb.[31][32][33]