It's a tail skid. The B-24 bombers were slung low to the ground, with very little clearance. When landing you had to nose up to touch the rear wheels first, but there was a chanse (pilot error, damage, gust of wind) that you'd slam your tail into the ground while landing. The skin kept that from happening, minimizing damage when it did happen. As you can see, the skid retracted when in flight.
EDIT: At least that's how I've always understood it.