 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.The initial raids were carried out by the Twentieth Air Force operating out of mainland China in Operation Matterhorn under XX Bomber Command, but these could not reach Tokyo.Operations from the Northern Mariana Islands commenced in November 1944 after the XXI Bomber Command was activated there.