Certain advancements during the 1920s in the design and technology of aircraft gave the United States a new role in the international sphere of aviation. Improvements in wind-tunnel testing, engine and airframe design, and maintenance equipment made for better-performing airplanes. As a result, private planes became less expensive and, in turn, grew in number and popularity.
The development of the autopilot can be traced back to 1908, when Elmer Sperry (1860–1930) introduced a type of gyrocompass that was later used in ship piloting systems (magnetic compasses were unreliable in steel-hulled ships). In 1929, Sperry's company tested a similar device for aircraft, as well as another device, the artificial horizon. These instruments, which enabled the pilot to fly without seeing the ground below, were rapidly installed aboard mail and commercial airplanes.
Important planes of the interwar period were Boeing's 247, introduced in 1933 and considered to be the first modern airliner. In order to compete, the Douglas Aircraft Company created its DC line: DC-1, DC-2, and DC-3. Boeing countered with its Model 307 Stratoliner. However, the DC-3, which went into service in 1936, is generally considered the first commercially popular (and profitable) plane. It had a number of innovative design features, including a retractable landing gear, and with twin 1,200-horsepower engines it could reach a maximum speed of 230 mph. A variant of the DC-3, the C-47, became the mainstay of the military's transport fleet in World War II.
Growing demand for passenger airline service soon pushed the aviation industry to even further advancements in passenger capacity and comfort, new elevation capabilities, and speed.