From 1920 to 1926, pilots on the Transcontinental Airmail Route faced Wyoming's high altitude, unpredictable weather, and severe winds as they flew through Cheyenne, and aviators including Slim Lewis, Hal Collison, Frank Yager, Harry Chandler, and Jack Knight became legends for their nerve, skill, and narrow escapes. After the original Cheyenne hangar burned in November 1924, five new hangars for the airmail service were built between Central and Warren Avenues just west of the current Administration Building. In 1925, Congress passed the Kelly Air Mail Act, allowing the Post Office to turn airmail over to civilian contractors, and by July 1927 Boeing Air Transport had taken over operations in Cheyenne, retaining the aviation facilities and many airmail pilots while establishing its main maintenance facility there. During this transition, Boeing Air Transport replaced the DeHavilland DH-4s that had served the routes for the previous seven years with lighter, more powerful Boeing B-40 aircraft that could more easily cross the state's mountain barriers and avoid its worst weather, while also carrying up to four passengers in a small cabin ahead of the pilot. In 1928, these aircraft were joined by the three-engined Boeing B-80 biplane airliner, which featured individual reading lights, leather seats, wood-paneled walls, and seating for 18 passengers, though flights remained loud and often cold. By 1929, B.A.T. had constructed the building now used for airport administration, housing ticketing, dispatch, communications, employee training, and weather monitoring, and in 1930 it built the hangar immediately west of the Administration Building to protect its aircraft as Boeing Air Transport, Pacific Air Transport, National Air Transport, and Varney Airlines merged to form United Air Lines. That same year, United trained eight young women as the world's first airline stewardesses and kept Cheyenne as its principal maintenance and training facility for the next seventeen years. In 1933, United introduced the Boeing B-247, the first modern airliner with a single mono-wing configuration and an all-metal skin, and in 1937 it added the Douglas DC-3, which could carry up to 35 passengers.