With the end of the First World War, the AEF's Air Service was dissolved and the Army Air Service in the United States largely demobilized. In 1920, the Air Service became a branch of the Army and in 1926 was renamed the Army Air Corps. During this period, the Air Corps began experimenting with new techniques, including air-to-air refueling and the development of the B-9 and the Martin B-10, the first all-metal monoplane bombers, and new fighters. Americans were fascinated with aviation in the 1920s and 1930s and refused to allow War Department conservatism to block innovation. General Billy Mitchell, the deputy director of the Air Service sought to wrest control of coastal defense away from the Navy. He went public insisting that his planes could sink battleships any day, a claim proven with a series of tests that culminated in the sinking of the "Ostfriesland". Mitchell lost his self-control in 1925 when he accused the Navy in a press release of "incompetency, criminal negligence and almost treasonable administration of the national defense." He received the highly publicized court martial he wanted, and was allowed to expound his theory that air power alone would suffice to win the next big war. He was convicted, and resigned. He became a popular hero and public opinion forced the War Department to strengthen the Air Corps. Mitchell's main argument was air power had to be autonomous—had to be controlled by fliers who understood the new technology, new tactics, new strategies, and who would not waste precious air assets in trying to assist old-fashioned armies and navies. Until his death in 1936 Mitchell, as a civilian, was a tireless prophet of airpower before numerous civilian audiences, but he lost touch with aviation developments and ceased to be influential inside the services. Indeed, his almost hysterical attacks made many generals hostile. The Air Corps managed a few publicity stunts, but always seemed to be overshadowed by glamorous civilians like Charles Lindbergh, Howard Hughes or Amelia Earhart. In 1934 President Franklin Roosevelt, feuding with the airline industry, suddenly turned the delivery of air mail over to the Air Corps. Multiple crashes by inexperienced Air Corps pilots in mediocre planes with poor navigation gear emphasized the fragility of the new service, and undercut its claims that in wartime it could perform miracles. Roosevelt, however, had become a firm believer in air power and had behind him both public opinion and Congress. When mobilization began in spring 1940 Roosevelt was as energetic as anyone in expanding the Air Corps role, calling for 50,000 planes a year, and sending the best new models to Britain for its war against the Luftwaffe.