In both the Civil War and the Spanish-American War, the United States made use of lighter-than-air balloons for observation purposes, but it was not until 1907 that the first effort was made to employ heavier-than-air devices of the type that the Wright Brothers had pioneered a few years earlier. It began on an extremely small scale, as the Aeronautical Division of the U.S. Army's Signal Corps. Soon thereafter, the Army asked for bids for a suitable plane, which was expected to carry two men for an hour at a speed of 40 mph. The Wright Brothers produced such a plane in 1909. They trained the first few pilots and soon the Army had its own training schools. In 1914, Congress gave official recognition to the Army's aeronautical efforts by establishing the Aviation Section of the Signal Corps, with an authorized strength of 60 officers and 260 men. Progress was made in using the airplane as element of combat, with the first attempts to drop bombs and fire machine guns from the air. Compared with the European powers, however, it was a minor effort and when the United States entered World War I, it could count only 35 Army pilots. The United States quickly made up for lost time. An Air Service was created, separate from the Signal Corps, and large appropriations made possible increased production of planes and the training of many pilots. When the Armistice was declared in 1918, America was deploying 45 squadrons at the front with 740 airplanes. There were 195,000 men serving in the Air Service. It took more than a year after the declaration of war before American planes actually entered combat in the European theater. American planes scored a disproportionately high number of enemy planes downed, and Billy Mitchell gained his first fame with massed bombing raids on German positions behind the lines. After the war, Mitchell used his bombers to demonstrate the ability of airplanes to destroy naval vessels. Having successfully demonstrated the results of air power, Mitchell promoted that use of airplanes are the primary military weapon of the future. His aggressive tone brought him into sharp conflict with the admirals and generals of more orthodox views and in 1925 he was courtmartialed. After being suspended, Mitchell resigned in temporary disgrace, although his views were borne out by later events. Despite Mitchell's failure, the rise in importance of air power was inexorable. In 1926, Congress raised its profile by adding an Assistant Secretary of War for Air and changing the name of the force from Air Service to Air Corps. Growth in numbers was slow, with only 45 squadrons in service in 1932. The realization that bombers could play a major role led to the development by the Boeing Company of a four-engine bomber. The first prototype of a heavily armored "Flying Fortress" flew in 1935. The approach of World War II occasioned a steady growth in Army air capabilities, which took a sharp upward turn after the fall of France in 1940. In June, 1941, all air units of the Army were absorbed into the Army Air Forces. Expansion accelerated after the United States entered the war following the attack on Pearl Harbor in December 1941. In 1942, the A.A.F. was made one of three main components of the Army, under the command of General H.H. Arnold. By 1944, with a strength of 2,386,000, the Army Air Forces constituted the largest air force in the world. During World War II, the A.A.F. operated in both the Europea and Pacific Theaters. In Europe, air power was instrumental in North Africa and in providing command of the skies during D-Day in Normandy. Both British and American bombers pounded German targets, with the Americans generally opting for daylight raids that allowed for greater precision. In the Pacific, Pearl Harbor was a great setback and the A.A.F. retreated along with other American military units. The Doolitle Raid on Tokyo had a great effect on wartime morale, although its military importance was less. Gradually supplied with forward air fields, bombing raids on Japan were undertaken with great effect and two A.A.F. B-29 bombers carried the atomic bombs that were dropped on Hiroshima and Nagasaki. In 1947, the military reorganization that produced the Defense Department created an independent branch of the military devoted to air power. The U.S. Air Force was given its own secretary, reporting to the Secretary of Defense. In addition to the Air Defense Command and the Tactical Air Command, the Strategic Air Command (SAC) was created to provide for the long-distance delivery of atomic and hydrogen bombs. The Korean War renewed the importance of the Air Force and introduced the first combat use of American jet aircraft. With the development of longer range missiles, the Air Force was given the responsibility for all intercontinental ballistic missiles or ICBMs.