First Appearance: Marvel Super-Heroes #16 (September 1968).
Appearances: Marvel Super-Heroes #16, Invaders #7, Incredible Hulk #135, Ghost Rider v2 #12.
Years Active: 1914-1918.
Karl Kaufman, an American-born pilot of German parents, wanted to use his skills as a pilot against the Germans in World War One, but was afraid that if he flew as himself, the Germans would punish his parents, who still lived in Germany. Therefore he put on a mask and cape and patrolled the skies over Europe during WW1 as the Phantom Eagle, probably the greatest pilot of the war. Later in the war, the Eagle joined the group the Freedom's Five and fought alongside them against various enemies, including Baron Blood.
Near the end of the war, the Eagle flew to Germany to begin fighting against the Germans as himself, rather than as the Phantom Eagle. He was discovered by a German pilot who gunned Kaufman and his parents down. Kaufman's spirit then began haunting the pilot, and pursued him across the world and the decades, finally bringing him to justice, with the help of the Ghost Rider, in the modern era.
Note: Todd Britton reminds me that Fawcett, during the 1940s, published the adventures of a heroic pilot named the Phantom Eagle, and that Roy Thomas probably based the Marvel version on the Fawcett version.
Note the date of the Phantom Eagle's creation. He may well be the first
Marvel retcon hero, well predating the Invaders. Of course, Nick
Fury was created in 1963, but he's more of an action/adventure
hero than a superhero.