The world's first regularly scheduled Air Mail Service began on May 15, 1918. That morning, pilots readied to fly mail between Washington, D.C., Philadelphia, Pennsylvania, and New York City. The army continued to manage the service for the next three months. In early August, the Post Office Department took over the airmail service and ran it through 1926.
Example of an envelope from the May 15, 1918 airmail flights.
Learn more about the Army pilots who flew the flights on May 15, 1918