Just adding more guns probably might not have made much difference. The B-17 was designed at a time well before the air corps had experience with operating bombers in the presence of modern enemy fighters, and at the time when armament standards were still being developed (and enforced on the manufacturers, not without difficulty).
As a result, the B-17's armament was a rather mixed affair, even on the redesigned E series and later models. The Sperry dorsal was a good turret, and the Sperry-Briggs ball turret was perhaps the best ventral turret in existence. The tail gun installation was primitive but effective. The waist and radio room gun installations were primitive and ineffective; their gunners faced a nearly impossible task aiming their guns manually at targets that passed fleetingly through their field of view. And the weak nose gun armament, and lack of armour there, was a major vulnerability of the B-17E and F.
That some redesign of stations and redistribution of guns could reduced weight and crew while actually improving the defense, was proven by the "Project Bovingdon" B-17, but this would have involved major redesign and an interruption of production.
Meanwhile, varying unofficial modifications were made to some aircraft: Including some that had 20-mm cannon as nose guns or tail guns. The nose gun was a one-off as far as I know, because the recoil damaged the nose structure and because the gunner found it difficult to handle; the tail cannon were more common and usually placed at the rear of the formation to deter enemy fighters with rockets or long-range guns.
The claims made by bombers' gunners for the destruction of enemy fighters were probably around a factor 10 too high, for numerous tactical, technical and psychological reasons. Many people knew this at the time and very few people believe these numbers now; but at the time they had propaganda value.
