The key development for the bombing of Japan was the B-29 Superfortress strategic bomber, which had an operational range of and was capable of attacking at high altitude above , where enemy defenses were very weak. Almost 90% of the bombs dropped on the home islands of Japan were delivered by this type of bomber. Once Allied ground forces had captured islands sufficiently close to Japan, airfields were built on those islands (particularly Saipan and Tinian) and B-29s could reach Japan for bombing missions.