By 1917, the U.S. Government had seen enough progress in the development of its air fleet that the administration decided to try something new:  air mail. To get the idea off the ground (literally), Congress appropriated $100,000 for an experimental airmail service.  The innovative service was under the joint control the Army and the Post Office.  It operated between Washington and New York, with an intermediate stop in Philadelphia. The first flight left Belmont Park, Long Island, for Philadelphia on May 14, 1918, and the next day continued on to Washington where it was met by President Woodrow Wilson. 

Re-Purposing War Planes – With a large number of war-surplus aircraft in hand, the Post Office almost immediately set its sights on a far more ambitious goal, which was transcontinental air service. It opened the first segment, between Chicago and Cleveland, on May 15, 1919, and completed the service on Sept. 8, 1920, when the most difficult part of the route, the Rocky Mountains, was spanned. Airplanes still could not fly at night when the service first began, so the mail was handed off to trains at the end of each day. Nonetheless, by using airplanes the Post Office was able to shave 22 hours off coast-to- coast mail deliveries.