In the early morning hours of August 6, 1945, a B-29 bomber named Enola Gay took off from the island of Tinian and headed north by northwest toward Japan. The bomber's primary target was the city of Hiroshima, located on the deltas of southwestern Honshu Island facing the Inland Sea. Hiroshima had a civilian population of almost 300,000 and was an important military center, containing about 43,000 soldiers. The bomber, piloted by the commander of the 509th Composite Group, Colonel Paul Tibbets, flew at low altitude on automatic pilot before climbing to 31,000 feet as it neared the target area. At approximately 8:15 a.m. Hiroshima time the Enola Gay released "Little Boy," its 9,700-pound uranium bomb, over the city. Tibbets immediately dove away to avoid the anticipated shock wave. Forty-three seconds later, a huge explosion lit the morning sky as Little Boy detonated 1,900 feet above the city, directly over a parade field where soldiers of the Japanese Second Army were doing calisthenics. Though already eleven and a half miles away, the Enola Gay was rocked by the blast. At first, Tibbets thought he was taking flak. After a second shock wave (reflected from the ground) hit the plane, the crew looked back at Hiroshima. "The city was hidden by that awful cloud . . . boiling up, mushrooming, terrible and incredibly tall," Tibbets recalled. The yield of the explosion was later estimated at 15 kilotons (the equivalent of 15,000 tons of TNT).