https://chroniclingamerica.loc.gov/lccn/sn86092536/1918-05-15/ed-1/seq-1/

President Wilson will receive the first airmail letter as service between Philadelphia, New York and Washington commences. A new $50 million ordnance factory is being built in the Ohio Valley, the Neville Island plant will be the largest in the world.

Source: "President to Receive First Air-Post Letter," The Wheeling Intelligencer, Wheeling, West Virginia, May 15, 1918. Page 1, Column 5.