Yeah, you do see some B-24s on Sky Captain's island base--and either blimps or airships there, too, that could be easily built. (There are currently both card and balsa models of these early airships. And of course some vintage plastic kits of same, if you shop around the vendors listed in FSM.) The B-24s are "J" or later models, with their nose gun turrets--but 1939 is too early for them for them if this was Our World, whcih its not. "J" and later B-24s wern't seen until the third year of WWII. Academy or Minicraft has several very nice late-model (1/72nd) B-24s in their line--or did have not too long ago. But of course this is Sky Captain's alternate retro-future, where the world of Superscience, as seen in the 1930s sci-fi pulp magazines, reigns supreme. (Great stuff!)
Some other modeling possibilities: The Hindenbeurg III from the beginning of the film--Notice how you're not quite sure if the Nazi's have come to power in this reality, as the III's tailfins are obscurred, as are the photos in the newspaper scene montage. It's unclear as to who's running Germany in Caps world. Are they Nazis? Revell-Germany's Hindenburg kit is still available. And theire are also several card and balsa models available of her now,, too.
And then there are those huge British flying aircraft carriers with their dual-prop, "Buck Rodgers" amfibious fighters. Or how about those flying robiots over NYC, that in the air look just like the robots in the second Max Fleisher Superman cartoon from the 1940s.
I can only hope that some eneterprising "gartage kit" company might produce some of these examples in the future. Just how many Trek and Star Wars kits can we build before deja vu (and boredom) sets in?
--Ken