On April 14, 1918, Douglas Campbell was the first US pilot to shoot down an enemy plane. He was also the first American to down 5 German planes making him the first American ace. From the article:
"American pilots engage in first dogfight over the western front Six days after being assigned for the first time to the western front, two American pilots from the U.S. First Aero Squadron engage in America’s first aerial dogfight with enemy aircraft. In a battle fought almost directly over the Allied Squadron Aerodome at Toul, France, U.S. fliers Douglas Campbell and Alan Winslow succeeded in shooting down two German two-seaters. By the end of May, Campbell had shot down five enemy aircraft, making him the first American to qualify as a “flying ace” in World War I.
The First Aero Squadron, organized in 1914 after the outbreak of World War I, undertook its first combat mission on March 19, 1917, in support of the 7,000 U.S. troops that invaded Mexico to capture Mexican revolutionary Pancho Villa. Despite numerous mechanical and navigational problems, the American fliers flew hundreds of scouting missions for U.S. Brigadier General John J. Pershing and gained important experience that would later be used over the battlefields of Europe in World War I."