Just looking at the length you give for the prop blade as 70.8 inches and allowing a little more for the part that is broken off that would have gone into the hub and the hub itself, I think you may have parts from a P-47. This site give the diameter of a P-47 propeller as originally 146 inches but then they increased the size to 13 feet (156 inches) and ended at 13 feet 2 inches in the final models. The props on B-17s and B-24s were smaller.
http://en.wikipedia.org/wiki/Republic_P-47_Thunderbolt