First Appearance: Adventures of Captain America #1 (September
Appearances: Adventures of Captain America #1-3.
Years Active: 1917-1941.
The American Eagle was Lieutenant Colonel James Fletcher. Fletcher was a hero of some renown during World War One; when Fletcher introduces himself to Steve Rogers during the Project: Rebirth trials, Rogers' reaction is ""Jim Fle---I know him! That's the American Eagle!" It is not known whether Fletcher was costumed or not, but from his own words he was a battlefield hero:
Have you ever seen human agony? Have you ever seen the carnage of a bloodied battlefield? Bullets rip into your friends? Metal fury shredding a child inches in front of you before you could rescue her? Have you ever seen the effects of mustard gas as it swept across an innocent European village? Those are the sights of war. Those are the sights I have seen.However, it would seem that Fletcher was not constantly on the front lines; when Rogers complains about being kept under wraps so the Germans won't get their hands on him, saying that he can't stand sitting around while brilliant men are dying, Fletcher responds with "I know how it feels. I know...I have walked a mile or two in your shoes." Presumably Fletcher was held back at least part of the time so as not to risk his death and thereby give the Germans a morale boost.
During World War Two he was the security chief and head "drill sergeant" for Project: Rebirth, the project which gave Captain America (I) his powers. Fletcher is captured by the agents of the Red Skull, but tears apart the chair he is bound to, dives through a window, fights off one of the Skull's agents, and briefly eludes being recaptured as he makes his way to a pay phone; clearly, being chained to a desk for the decades between the years did not slow him down much. He is eventually recaptured and held prisoner in Paris; he withstands torture and drugs before leaping through a window (as a way, I assume, to avoid breaking under torture, or perhaps just to escape) and falling to his (presumed) death on the sidewalk below.
Note: My guess is that he is the Marvel version of the American Eagle of the Golden Agency, the World War Two group on the Earth of Squadron Supreme. But this has not been proven.