First US aviator, assigned to a US unit at the time he was shot down, whom I can find was Ensign Albert Dillon Sturtevant, USNR, shot down and killed on 15 February 1918 over the North Sea. Which makes him also the first US naval aviator killed in action. Sturtevant was assigned to the USN patrol detachment located at Felixstowe. Posthumous Navy Cross.
Rich