Seventy years ago last August, a B-29 bomber named the Enola Gay released its 4,000kg load over the Japanese city of Hiroshima, the sudden loss of weight jolting the US aircraft violently upwards as the pilot banked hard to escape the imminent blast. “My God, what have we done,” wrote Enola Gay co-pilot Robert Lewis, recalling in his journal the morning of August 6, 1945, when he witnessed the atomic bomb, code-named Little Boy, successfully detonate 1,800 feet above Hiroshima.