Although the American experience sometimes reflected European trends, it also demonstrated clear differences. Under the auspices of the U.S. Post Office, an airmail operation was launched in 1918 as a wartime effort to stimulate aircraft production and to generate a pool of trained pilots. Using Curtiss JN-4H (“Jenny”) trainers converted to mail planes, the early service floundered. After the war, shrewd airmail bureaucrats obtained larger American-built De Havilland DH-4 biplanes with liquid-cooled Liberty engines from surplus military stocks. Their top speed of 80 miles (130 km) per hour surpassed the 75 miles (120 km) per hour of the Jenny, allowing mail planes to beat railway delivery times over long distances. By 1924, coast-to-coast airmail service had developed, using light beacons to guide open-cockpit planes at night. Correspondence from New York now arrived on the west coast in two days instead of five days by railway. This savings in time had a distinct impact on expediting the clearance of checks, interest-bearing securities, and other business paper with a time-sensitive value in transfer between businesses and financial institutions.

Having established a workable airmail system and a considerable clientele, the Post Office yielded to congressional pressures and, with the Contract Air Mail Act of 1925, turned over the mail service to private contractors. The following year, the Air Commerce Act established a bureau to enforce procedures for the licensing of aircraft, engines, pilots, and other personnel. The former act stimulated design and production of advanced planes to compete with rival carriers; the latter reassured insurance companies, private investors, and banks that safety standards would be enforced. With these elements in hand, American aviation rapidly progressed. Ironically, at the same time that European countries organized subsidized national flag lines and followed practices that often discouraged innovation in the design of airliners, the United States turned over civil aviation to commercial operators, where aggressive competition accelerated significant developments in aviation technology and aircraft performance.

For one thing, manufacturers of airplane motors began a significant period of development in modern piston engines. Because liquid-cooled in-line engines offered less frontal surface, they were often favoured by military designers. With these engines, aircraft could be streamlined to improve speed but with a trade-off in complexity and weight because of the requisite coolant, coolant lines, radiator, and associated pumps. Air-cooled radial designs, in contrast, achieved relative simplicity, reliability, and comparatively light weight at the cost of more air resistance (creating drag) because of their blunt shape. In 1928, the National Advisory Committee for Aeronautics (NACA) announced its famous cowling for radial engines. It not only smoothed airflow around the engine, substantially reducing drag, but also enhanced the cooling of the cylinders. With their dependability and ease of maintenance, radial engines became the type most favoured by designers of American air transports. The Curtiss-Wright Corporation (formed from the merger of Curtiss Aeroplane and Motor Company and Wright Aeronautical in 1929) produced a series of Whirlwind and Cyclone radial engines; Pratt & Whitney Aircraft launched its Wasp designs. Many of these American radial engines powered airplanes built overseas. By the end of the 1930s, innovations such as variable-pitch propellers, superchargers (to enhance high-altitude engine performance), and high-octane fuels had contributed to dramatically improved performance in both liquid-cooled and air-cooled radial engines.

During the late 1920s and early 1930s, the U.S. Post Office instituted payment formulas that favoured aircraft large enough to carry passengers as well as mail. A rising volume of research reports from the NACA facilitated many improved aircraft designs. The result was a swift increase in larger planes with improved radial engines and a shift from biplanes to trimotor monoplane transports marketed by a subsidiary of Ford and by the European builder, Anthony Fokker, who had set up shop in the United States.

Largely owing to airline rivalry, American technology had already taken a major step forward with the introduction of the Boeing Company Model 247 airliner, which cruised at about 180 miles (290 km) per hour and entered service with United Airlines, Inc., in 1933. With its all-metal stressed-skin construction (which used the metal skin covering itself to carry aerodynamic loads), retractable gear, two 550-horsepower Pratt & Whitney Wasp radial engines, and cowlings inspired by NACA research, the 10-passenger Model 247 seemed to be head-and-shoulders above competitive aircraft.

Shortly before the 247 began flying, a Fokker trimotor of Transcontinental & Western Air, Inc. (TWA), crashed in a Kansas farm field. Everybody aboard died, including the University of Notre Dame’s revered football coach Knute Rockne. Subsequent investigation of the crash raised questions about structural weakness in the plane’s main wooden-wing spar. Controversy about “wooden airplanes” and criticism of the Fokker plane generally gave trimotor airliners a bad image. When TWA asked manufacturers to submit designs for a replacement, Douglas Aircraft Company (later McDonnell Douglas Corporation) responded with an all-metal twin-engine airliner. The DC-2, with an advanced NACA cowling, refined streamlining, and other improvements, mounted Wright Cyclone engines and carried 14 passengers, surpassing the Boeing 247 in every way. Significantly, leading European airlines such as KLM acquired the new Douglas transport, beginning a trend for European operators to buy American equipment. A subsequent model, the legendary DC-3, entering service in 1936, mounted 1,000-horsepower Cyclone or Wright Wasp radial engines, cruised at 185 miles (300 km) per hour, and carried 21 passengers—double the capacity of the Boeing 247. By 1939, with superior seat capacity, performance, and ancillary refinements, DC-3 transports already were carrying 90 percent of the world’s airline traffic.

While the Douglas transports dramatically improved air travel within the United States and along European routes, airline entrepreneurs kept looking for a vehicle for transoceanic travel. Many in the 1930s still believed that huge gas-filled airships would be the key. Germany built diesel-powered hydrogen-filled airships, or dirigibles, such as the Hindenburg, which flew North Atlantic schedules between Europe and the United States during the summer months. American Airlines, Inc., publicized special schedules that allowed DC-3 passengers to make transatlantic connections with the Hindenburg’s terminus in New Jersey. This short-lived arrangement ended with the Hindenburg’s tragic and fiery destruction upon its arrival from Europe to open the 1937 travel season. Plans for utilizing dirigibles as passenger liners quickly faded.

That left flying boats. Pan American World Airways, Inc. (Pan Am), purchased a number of designs from the Russian-born American engineer Igor Sikorsky. Pan Am operated them on overwater routes in the Caribbean region, often saving weeks of travel time when compared with steamship and railway connections. By the late 1930s, American manufacturers such as the Martin Company (now the Martin Marietta Corporation), Sikorsky, and Boeing were all producing very large four-engine flying boats intended for service over the Atlantic and Pacific. In 1935, using islands strung across the Pacific, Pan Am completed installation of stopover passenger facilities and its own radio communications and meteorological network. With Martin flying boats, most flights carried mail, along with occasional government or business passengers who could pay the high fares. Inaugural departures occasioned considerable fanfare. In 1939 Eleanor Roosevelt, wife of U.S. President Franklin D. Roosevelt, smashed a bottle of champagne over the bow of an imposing Yankee Clipper flying boat to launch scheduled airmail and luxury passenger service across the Atlantic to Europe. These promising, if expensive, travel innovations were soon curtailed by wartime conditions in Asia and Europe. In any case, progress in long-range land-based four-engine airliners represented advanced engineering that would have soon displaced flying boats.

Fortuitously, the widespread boundaries of the United States contained a growing number of urban complexes with intervening distances that made airline service a desirable option. American transport designs tended to favour more speed for time-conscious passengers. In comparison, airways within the closer boundaries of western Europe favoured short-haul service, often trading speed for luxury, even on longer colonial routes where state subsidies deflected technological competition.

Boeing’s Stratoliner, a pathbreaking transport that featured a pressurized cabin, entered service in 1940. Pressurization enabled airliners to fly above adverse weather, permitting transports to maintain dependable schedules and giving passengers a more comfortable trip. Moreover, at higher altitudes, airliners actually experienced less atmospheric friction, or drag, enhancing their performance and fuel efficiency. Only a few Stratoliners entered service before World War II led Boeing to focus on building bombers.

Meanwhile, Douglas had introduced the DC-4. Although it was unpressurized, it possessed a comparable performance to the Stratoliner and could carry more passengers. Also, the DC-4 had a tricycle landing gear (unlike the Stratoliner’s conventional tail wheel), which facilitated boarding of passengers, improved the pilots’ view of the runway and surrounding airport environment, and enhanced the plane’s takeoff characteristics. The DC-4 achieved production status (as the C-54) during the war as the U.S. Army Air Forces’ principal long-range transport. Late in the conflict, it was joined by the Lockheed L-049 Constellation (instantly identifiable by its triple vertical fins), originally designed in 1939 as a commercial airliner that blended a pressurized fuselage, tricycle landing gear, and other state-of-the-art features. Characteristics of these sophisticated civilian planes gave the United States a major advantage in postwar airliner competition.