Post Office Department Map of Continental U.S. Air Mail Routes

In 1918 Post Office pilots flying War Department planes began to deliver mail between New York City and Washington, DC, with a stop in Philadelphia. Later in the year, the Post Office assumed complete direction of airmail services between New York City and Washington, and by 1924 the Post Office offered to the public airmail service between New York City and San Francisco, CA. The Air Mail Act of 1925 established a contract system whereby private companies provided airmail service along routes that connected with the government-operated transcontinental route. Reflecting the rapid expansion of airmail use, the price for an ounce of mail dropped from $0.10 per half ounce in 1927 to $0.05 an ounce by 1928.