First off damn you for linking to Tv Tropes as I needed to get stuff done today. Second off i'll take the B-17 for 500 Alex. Nothing in the war could take more punishment and still bring the crew back home.
I'll second the vote for the B-17, but I'm taking it a step further.
Proved to be totally worthless in actual combat, but for sheer awesomness, I've got to go with the gunship version of the B-17, the YB-40.
First, you take the already awesome and indestructible frame of the B-17. Get rid of all that silly bomb carrying stuff.
Then, you slap on more .50cal turrets, loading the thing up with: "...18 (or more) × .50 in (12.7 mm) Browning M2 machine guns."
A B-17, looking like a friggen porcupine of death is about the most badass thing I can image.