use of Zeppelins and, later on, bombers such as the Gotha, to drop bombs on Southern England. By the end of the war airplanes had become specialized into bombers, fighters, and observation (reconnaissance) aircraft. By 1916, aerial combat had already progressed to the point where dogfighting tactics based on such doctrines as the Dicta Boelcke allowed air supremacy to be achieved. During the course of the war, new designs led to air supremacy shifting back and forth between the Germans and Allies. Interwar period Between 1918 and 1939 aircraft technology developed very rapidly. In 1918 most aircraft were biplanes with wooden frames, canvas skins, wire rigging and air-cooled engines. Biplanes continued to be the mainstay of air forces around the world and were used extensively in conflicts such as the Spanish Civil War. Most industrial countries also created air forces separate from the army and navy. However, by 1939 military biplanes were in the process of being replaced with metal framed monoplanes, often with stressed skins and liquid-cooled engines. Top speeds had tripled; altitudes doubled; ranges and payloads of bombers increased enormously. Some theorists, especially in Britain, considered that aircraft would become the dominant military arm in the future. They imagined that a future war would be won entirely by the destruction of the enemy's military and industrial capability from the air. The Italian general Giulio Douhet, author of The Command of the Air, was a seminal theorist of this school, which has been associated with Stanley Baldwin's statement that "the bomber will always get through"; that is, regardless of air defences, sufficient raiders will survive to rain destruction on the enemy's cities. This led to what would later be called a strategy of deterrence and a "bomber gap", as nations measured air force power by number of bombers. Others, such as General Billy Mitchell in the United States, saw the potential of air power to augment the striking power of naval surface fleets. German and British pilots had experimented with aerial bombing of ships and air-dropped torpedoes during World War I with mixed results. The vulnerability of capital ships to aircraft was demonstrated on 21 July 1921 when a squadron of bombers commanded by General Mitchell sank the ex-German battleship SMS Ostfriesland with aerial bombs; although the Ostfriesland was stationary and defenseless during the exercise, its destruction demonstrated the potency of air planes against ships. It was during the