One of the things which strikes me when I read some of these wartime comic books is how frequently Hitler himself is presented as a buffoonish foil - almost comic relief - rather than serious threat. I certainly understand why comics weren't willing to make Hitler himself out to be a credible threat but it reminds me of an observation John Cleese once made with regards to the difference between US perception of the Nazis and the UK. In America, WWII/Hitler is associated with The Holocaust and all of the horrors that conjures up in the mind; in the UK on the other hand, Britons thought of Hitler as that silly, little man and WWII was, for a time, thought of as yes, a major battle with major casualties, but a battle nonetheless. I don't know why this is - perhaps seeing the war first hand meant the English saw the damage to their own country as the definition of WWII while in America, the concentration camps which were liberated became the most tangible and horrific evidence of just what the Nazis were doing the whole time. What I'm getting at is this - it seems that during the war itself, Hitler wasn't necessarily thought of as he is today - then (and this is speculation on my part) he was more of a guy who lusted for land and power whereas now, we know him to be a sadistic, evil monster unlike any we've seen since. Not that his bigotry would have been much of a secret, but its interesting to read something like Captain America #2 "Trapped in the Nazi Stronghold" where Cap and Bucky spend much of the issue pulling pranks on Hitler ("Germans find Hitler standing in ashcan" reads one newspaper headline in the tale) as if he's Elmer Fudd. I believe there's also a Will Eisner Spirit story out there in which Hitler comes to America out of a desire to simply be liked for who he is and only wants a friend - hardly the type of depiction you'd expect for the man responsible for The Holocaust. In other words, Hitler doesn't seem to have been regarded as a symbol for bigotry, feelings of racial superiority, intolerance of those who were different from one another, so much as he seems to have been thought of as a warlord like any other without anything other than a lust for power motivating him.
As for further suggestions, how about possibly the most obvious one? Look Magazine's 1940 two page spread on "How Superman Would Win the War"?