The B-17 has very limited amounts of armour. Most of that was armoured glass to make the gunners and pilots feel a bit better looking through armored glass. The only other parts that had any real "armour" were the fuel tanks, but that was more the quality of the fabrics and gels used to seal them. Some of the B-17's had aluminum shells over their oil tanks and some of the inboard fuel tanks as well. This was more designed for splinter protection that anything else.
The thing to keep in mind is that the B-17 fuselage was made up of aluminum sheet over thin stamped ribs, stringers, and longerons. The only real strength of the fuselage is in the bomb bay where a sort of "box" is made up of "square aluminum tube" that provides the main area of strength and support in the aircraft. Urchin mentions that "empty space" was like the armour on a 17 and he is very correct. The B-17 is an unpressurized aircraft and that helps in preventing the blow out associated with pressurization. The amount of space and redundancy in design allows for an amazing capacity to literally absorb massive amounts of damage. But, the B-17 is also fragile when it comes to it's control system, which is comprised of woven cables.
Anyways, sorry for the diatribe, I spent a lot of time rebuilding one years back and became very intimate with it's construction.
