From the 1930s through the 1950s, Cheyenne's airport was a major center of aviation innovation and wartime aircraft modification. The first de-icing equipment was installed on aircraft there, and wide-scale installation of variable-pitch propellers, cabin heaters, and on-board air to ground capable radios began there as well. United's mechanics in Cheyenne became so proficient in innovation and problem solving that observers came from Britain, France, Soviet Russia and Nazi Germany, and Cheyenne was at one time considered the largest aviation roundhouse in the world. After the United States entered World War II in 1941, United used its technical expertise in Cheyenne for the war effort, first modifying two Boeing B-17 bombers in late December 1941 to carry extra fuel and cameras for aerial photography, then modifying eight more B-17s for the British; after the war, United officials learned that the first two planes had been used in reconnaissance flights over Tokyo before the Doolittle bombing raid of 1942. During World War II, Bomber Modification Center #10 in Cheyenne was highly active, and between January 2, 1942, and July 31, 1945, it retrofitted 5,736 Boeing B.17 bombers with equipment the factory could not install because of heavy production schedules. The airport expanded with two new hangars in 1942 and a third in 1943, and by the spring of 1943 the center employed 1,600 workers while an average of six planes a day left Cheyenne for combat areas. In 1942, United also moved its flight training school from California to Cheyenne, where almost 100 students trained year round. After the war, most of the hangar and rework facilities on the north side of the airport went to the Wyoming Air National Guard, and the former Boeing/United Headquarters Building became the city's Airport Administration Building. Because commercial airplanes remained unpressurized until the mid-1950s and most could not fly directly west from Denver over the Rocky Mountains, Cheyenne remained a major regional airfield and was often used in bad weather for layovers until flights could resume. United's stewardess school stayed in Cheyenne until about 1956, when it moved to Chicago. The Boeing/United Airlines Terminal, hangar and Airport Fountain in Cheyenne were built for BAT between 1929 and 1934, and their Louis Sullivan-influenced designs formed a consistent theme when Cheyenne Municipal Airport was a major air transport facility; the 1930 hangar was designed by Cheyenne architect Frederic H. Porter, and the 1934 Art Deco fountain was designed as a memorial to early aviation history.