To answer Blackace question about the yb40. It was a heavy fighter concept to escort bomb carrying b17s They were not equipped with the ability to drop bombs as there bomb bay was converted into an ammo magazine. The big changes were addition of the radio operator machines gun beiing replaced by a turret with two fifty in it , the waist gunner positions were staggered for better movement and there single fifty was replaced by two . the tail gunners position was redesigned a chin turret was added and the cheek guns were removed in the United States but returned once they get to England. it had double the armor of a standard b17. Performance so as you might have guessed suffered they were unable keep with bomb carrying b17s. The only things that were kept from this were chin turret and the tail redesign