The key development for the bombing of Japan was the B-29 Superfortress, which had an operational range of 1,500 miles (2,400 km); almost 90% of the bombs (147,000 tons) dropped on the home islands of Japan were delivered by this bomber. The first raid by B-29s on Japan was on 15 June 1944, from China. The B-29s took off from Chengdu, over 1,500 miles away. This raid was also not particularly effective: only forty-seven of the sixty-eight bombers hit the target area.