Domestic U.S. Air Mail was established as a new class of mail service by the United States Post Office Department (POD) on May 15, 1918, with the inauguration of the Washington–Philadelphia–New York route. Special postage stamps were issued for use with this service. Domestic air mail became obsolete in 1975, and international air mail in 1995, when the USPS began transporting First Class mail by air on a routine basis. All post-1977 United States stamp images are copyright of USPS.