On 6 August 1945, a Boeing B-29 Superfortress of the 393d Bombardment Squadron, piloted by Tibbets, lifted off from North Field, and Little Boy in its bomb bay. Hiroshima, the headquarters of the 2nd General Army and Fifth Division and a port of embarkation, was the primary target of the mission, with Kokura and Nagasaki as alternatives. With Farrell's permission, Parsons, the weaponeer in charge of the mission, completed the bomb assembly in the air to minimize the risks during takeoff. The bomb detonated at an altitude of 1,750 feet (530 m) with a blast that was later estimated to be the equivalent of 13 kilotons of TNT.An area of approximately 4.7 square miles (12 km2) was destroyed. Japanese officials determined that 69% of Hiroshima's buildings were destroyed and another 6–7% damaged. About 70,000 to 80,000 people, of whom 20,000 were Japanese combatants and 20,000 were Korean slave laborers, or some 30% of the population of Hiroshima, were killed immediately, and another 70,000 injured.
