There's plenty of other wartime superheroes. Even if you only go with overtly American ones and ignore the Human Torch, Submariner etc, you have ones like Uncle Sam, Lady liberty, Liberty Belle, the Shield and many others.
There's Sgt. Fury and his Howling Commandos. The Haunted Tank, the Unknown Soldier. I wouldn't use Sgt. Rock or many of the other DC war heroes because they tend to be more realistc than pulpy (except for the ones already mentioned). Marvel characters are better suited.
Don't forget the Blackhawks!