The U.S. aviation industry took off in the early 20th century not by transporting people, but by moving America’s mail. At first, airmail pilots flew in flimsy open-cockpit planes through every kind of weather—an experience that ranged from frequently harrowing to sometimes fatal. As routes expanded, airports proliferated and safer, more reliable aircraft were developed, the commercial air travel industry emerged, initially by piggybacking on airmail flights. The earliest passengers unceremoniously used mailbags for seats.

U.S. Mail first took to the skies less than a decade after the Wright Brothers made their pioneering flights over the dunes of Kitty Hawk. In 1911, the U.S. Post Office Department began staging dozens of experimental airmail flights at air meets, fairs and carnivals. On May 15, 1918, it launched the first scheduled service between New York City and Washington, D.C.

Initially, the U.S. Army Signal Corps operated the airmail route as a way to train its fledgling aviators before deploying them to the skies over Europe in World War I. That plan, however, immediately encountered turbulence.

On the inaugural flight from the nation’s capital, Lieutenant George Boyle’s plane failed to start because of an empty fuel tank. Then when the rookie pilot finally lifted off, he flew in the wrong direction and damaged his Curtiss JN-4H “Jenny” while landing in a freshly plowed field in an attempt to ask a farmer for directions. Boyle—and the mail—rode in a truck back to Washington, D.C. Two days later, the pilot became lost again and made an emergency landing at the Philadelphia Country Club after running out of fuel.

In August 1918, the Post Office took over the airmail service with civilian pilots and six specially built planes. Routes soon spread beyond the Northeast, stretching from coast to coast by 1924.

Without radio communications or reliable instruments, pioneering airmail pilots relied on landmarks and instincts to guide their fragile biplanes from city to city, sometimes as sleet lashed their faces and rain blurred their vision in open cockpits. “Flying at 30 to 50 feet with never over 100 feet forward visibility in the average fog—made a great many angels of good pilots,” wrote aviator Jack Knight. According to the U.S. Postal Service, nearly three-dozen airmail pilots died in crashes between 1918 and 1927.

After proving airmail’s financial viability—and building a transcontinental airway system with landing strips, beacons and even enormous concrete arrows pointing pilots in the correct direction—the Post Office in 1925 started taking bids from commercial aviation companies to provide airmail services.

The airmail contracts attracted some of the country’s most prominent business titans and aviators. The short-lived, but influential Ford Air Transport, owned by Henry Ford and his son Edsel, began the first commercial airmail service on February 15, 1926, on routes from Detroit to Cleveland and Chicago—flying Ford’s “Tin Goose,” the first American metal-clad, multi-engine plane envisioned primarily for passenger use. Two months later, Robertson Aircraft Corporation’s chief pilot, Charles Lindbergh, launched an airmail service between St. Louis and Chicago a year before his famous crossing of the Atlantic Ocean. Bad weather forced him to parachute to safety twice while flying that airmail route.

Further west, airmail helped William E. Boeing builds his aeronautical empire. With his airplane manufacturing company struggling to survive after the cancellation of military contracts at the end of World War I, Boeing and his lead test pilot, Eddie Hubbard, flew a bag of 60 letters from Vancouver, British Columbia, to Seattle on March 3, 1919, in North America’s first international airmail flight.

“Boeing envisioned a great future for the airplane beyond military use,” says Michael Lombardi, senior corporate historian for The Boeing Company. “Flying that airmail reinforced the reality that the airplane could have a very practical use to carry passengers as well as freight.”

Hubbard flew the first international contract mail route between Seattle and Victoria, British Columbia, and in 1927 lobbied Boeing to successfully bid on the country’s longest airmail route between San Francisco and Chicago. By September 1, 1927, all airmail transportation had been handed off to private companies. By 1929 more than 30 different airlines were delivering mail.

On May 23, 1926, a carrier called Western Air Express loaded not just mailbags—but two passengers—on each of its regular airmail flights between Los Angeles and Salt Lake City. The flight—which lasted seven-plus hours, including a stop in Las Vegas—was hardly first-class as the passengers sat atop mail sacks, ate a box lunch and were given tin cups to use in the absence of lavatories.

Service further expanded when Postmaster General Walter Folger Brown, operating with enhanced powers under the Airmail Act of 1930, masterminded a series of airline mergers and awarded passenger and airmail routes to the industry’s largest entities such as American Airways (formerly Robertson Aircraft Corp.) and United Air Lines, which emerged after Boeing consolidated smaller airlines he had purchased into a transcontinental carrier.

Following the election of President Franklin D. Roosevelt, however, charges of corruption in Brown’s awarding of the contracts led to a congressional investigation and the cancellation of all domestic airmail contracts on February 9, 1934. (The U.S. Court of Claims later determined that no fraud had actually occurred.) Roosevelt ordered the U.S. Army to transport airmail—with disastrous results as 10 pilots died in crashes during the next two weeks due to severe winter weather and inadequate equipment. In all, the Army Air Corps logged 66 crashes and 12 deaths.

Using its antitrust powers, the federal government barred aircraft manufacturers from owning airlines and pressured Boeing to dismantle his vertically integrated aviation conglomerate, United Aircraft and Transport. Upset at the decision, Boeing sold his stock and retired to spend more time at horse tracks and fishing off his yacht. “Boeing was very open and honest in all his business dealings,” Lombardi says, “and it crushed him to be treated as a criminal by the government.”

By the spring of 1934, airmail delivery returned to private companies—but not to any of those given contracts by Brown. That decision led to an industry reorganization and a spate of spinoffs and name changes that created some of the country’s best-known airlines—such as Northwest, Eastern, TWA, Continental and American—that went on to fly tens of millions of passengers.