Everett was on a roll in Feb 1940!
I always thought this was the 2nd swastika cover but it looks like it might be tied for second.
Personally, I think the first swastika/anti-nazi cover was by Charles Mazoujian, although the design of the swastika was either intentionally or accidentally misrepresented.
Moreover, this cover depicts coordinated carpet bombing, which was a Luftwaffe innovation (Condor Legion) first employed during the Spanish Civil War...
I'm sure the dude I originally acquired this sweet book from would concur.
This MMC 2 was probably the 1939 cover date book I was thinking of, although--as you note--it's a trifle ambiguous because the symbol on the plane is not quite a swastika. I wonder whether the cover was inspired by the terror bombing of Warsaw in September 1939, although the cover was probably already drawn prior to that assuming the book was on the stands a couple of months prior to its cover date.
One thing I've always thought about this cover: Your city is getting the krap bombed out of it and a super hero arrives to help ... and he manages to take down maybe one plane?
The interior story does involve Poland getting bombed.