The bombing

For the composition of the USAAF mission see 509th Composite Group.

Seizo Yamada's ground level photo taken from approximately 7 km northeast of Hiroshima.
Hiroshima was the primary target of the first nuclear bombing mission on August 6, with Kokura and Nagasaki being alternative targets. August 6 was chosen because there had previously been cloud cover over the target. The 393d Bombardment Squadron B-29 Enola Gay, piloted and commanded by 509th Composite Group commander Colonel Paul Tibbets, was launched from North Field airbase on Tinian in the West Pacific, about six hours flight time from Japan. The Enola Gay (named after Colonel Tibbets' mother) was accompanied by two other B29s, The Great Artiste which carried instrumentation, commanded by Major Charles W. Sweeney, and a then-nameless aircraft later called Necessary Evil (the photography aircraft) commanded by Captain George Marquardt.[18]
After leaving Tinian the aircraft made their way separately to Iwo Jima where they rendezvoused at 2,440 m (8,000 ft) and set course for Japan. The aircraft arrived over the target in clear visibility at 9,855 m (32,000 ft). On the journey, Navy Captain William Parsons had armed the bomb, which had been left unarmed to minimize the risks during takeoff. His assistant, 2nd Lt. Morris Jeppson, removed the safety devices thirty minutes before reaching the target area.[19]

Hiroshima, in the aftermath of the bombing
The release at 08:15 (Hiroshima time) was uneventful, and the gravity bomb known as "Little Boy", a gun-type fission weapon with 60 kg (130 pounds) of uranium-235, took fifty-seven seconds to fall from the aircraft to the predetermined detonation height about six hundred meters (1,900 ft) above the city. Due to crosswind, it missed the aiming point, the Aioi Bridge, by almost eight hundred feet and detonated directly over Shima Surgical Clinic.[20] It created a blast equivalent to about 13 kilotons of TNT. (The U-235 weapon was considered very inefficient, with only 1.38% of its material fissioning.)[21] The radius of total destruction was about one mile (1.6 km), with resulting fires across 11.4 km² (4.4 square miles).[22] Infrastructure damage was estimated at ninety percent of Hiroshima's buildings being either damaged or completely destroyed.
About an hour before the bombing, Japanese early warning radar detected the approach of some American aircraft headed for the southern part of Japan. An alert was given and radio broadcasting stopped in many cities, among them Hiroshima. At nearly 08:00, the radar operator in Hiroshima determined that the number of planes coming in was very small—probably not more than three—and the air raid alert was lifted. To conserve fuel and aircraft, the Japanese had decided not to intercept small formations. The normal radio broadcast warning was given to the people that it might be advisable to go to air-raid shelters if B-29s were actually sighted, but no raid was expected beyond some sort of reconnaissance.