and surveillance aircraft. Most of these airplanes were biplanes with wooden frames, canvas skins, wire rigging and air-cooled engines. Between 1918 and 1939, aircraft technology developed very rapidly. 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 (and oxygen masks become commonplace); ranges and payloads of bombers increased enormously. Some theorists, most famously Hugh Trenchard and Giulio Douhet, believed that aircraft would become the dominant military arm in the future, and argued that future wars would be won entirely by the destruction of the enemy's military and industrial capability from the air. This concept was called strategic bombing. Douhet also argued in The Command of the Air (1921) that future military leaders could avoid falling into bloody World War I-style trench stalemates by using aviation to strike past the enemy's forces directly at their vulnerable civilian population, which Douhet believed would cause these populations to rise up in revolt to stop the bombing. Others, such as Billy Mitchell, saw the potential of air power to neutralize the striking power of naval surface fleets. Mitchell himself proved the vulnerability of capital ships to aircraft was finally in 1921 when he commanded a squadron of bombers that sank the ex-German battleship SMS Ostfriesland with aerial bombs. (See Industrial warfare#Naval warfare) During WWII, there was a debate between strategic bombing and tactical bombing. Strategic bombing focused on targets such as factories, railroads, oil refineries, and heavily populated areas such as cities and towns, and required heavy four-engine bombers carrying large payloads of ordnance or a single heavy four-engine bomber carrying a nuclear weapon flying deep into enemy territory. Tactical bombing focused on concentration of combatants, command and control centers, airfields, and ammunition dumps, and required attack aircraft, dive bombers, and fighter bombers that could fly low over the battlefield. In the early years of WWII, the German Luftwaffe focused on tactical bombing, using large numbers of Ju 87 Stukas as "flying artillery" for land offensives. Artillery was slow and required time to set up a firing position, whereas aircraft were better able keep up with the fast advances of the German panzer columns. Close air support greatly assisted in the successes of the German Army in the Battle of France. It was also important in amphibious warfare, where aircraft carriers could provide support