At the same time, a brand-new role was emerging. As early as 1911, airplanes had been used to fly the mail, and it didn't take long for the business world to realize that airplanes could move people as well. The British introduced a cross-channel service in 1919 (as did the French about the same time), but its passengers must have wondered if flying was really worth it. They traveled two to a plane, crammed together facing each other in the converted gunner's cockpit of the De Havilland 4; the engine noise was so loud that they could communicate with each other or with the pilot only by passing notes. Clearly, aircraft designers had to start paying attention to passenger comfort.
The result was a steady accumulation of improvements, fostered by the likes of American businessman Donald Douglas, who founded his own aircraft company in California in 1920. By 1933 he had introduced an airplane of truly revolutionary appeal, the DC-1 (for Douglas Commercial). Its 12-passenger cabin included heaters and soundproofing, and the all-metal airframe was among the strongest ever built. By 1936 Douglas's engineers had produced one of the star performers in the whole history of aviation, the DC-3. This shiny, elegant workhorse incorporated just about every aviation-related engineering advance of the day, including almost completely enclosed engines to reduce drag, new types of wing flaps for better control, and variable-pitch propellers, whose angle could be altered in flight to improve efficiency and thrust. The DC-3 was roomy enough for 21 passengers and could also be configured with sleeping berths for long-distance flights. Passengers came flocking. By 1938, fully 80 percent of U.S. passengers were flying in DC-3s and a dozen foreign airlines had adopted the planes. DC-3s are still in the air today, serving in a variety of capacities, including cargo and medical relief, especially in developing countries.
Improvements in the mechanisms of control and in airframe construction continued, driven by commercial considerations and, with the advent of World War II, military demands for bigger bombers and faster and more maneuverable fighters. Aviation's next great leap forward, however, was all about power and speed. In 1929 a 21-year-old British engineer named Frank Whittle had drawn up plans for an engine based on jet propulsion, a concept introduced near the beginning of the century by a Frenchman named Rene Lorin. German engineer Hans von Ohain followed with his own design, which was the first to prove practical for flight. In August 1939 he watched as the first aircraft equipped with jet engines, the Heinkel HE 178, took off.