A timely followup to this post, here is a cutaway illustration of the Consolidated Vultee Model 36 (development phase of the B-36) dated 4-19-1944. The fuselage and tail appear to be pretty much what the XB-36 wound up with, but the defensive armament is still a ways off. In the top and bottom forward gun bays are manned, pressurized, retractable turrets each packing two 37-mm cannon. The aft top and bottom gun bays each have a single remotely controlled turret each with four 50 caliber machine guns.
I can’t help but think that “pressurized, retractable turrets” would have been another way of saying “design headache, maintenance nightmare.”