During WW2, there were two different definitions for a gunship. The first was a more heavily armed bomber, meant to protect other bombers in a formation from enemy fighters. There was a B-17 variant, called the YB-40, used in this role. It had a second dorsal turret and the first instance of the "chin gun" which later found its way to other models, among other improvements. The second was the modern definition of a gunship; a dedicated CAS aircraft, using its guns to pound ground targets either in strafing attacks or in a pylon turn. As far as I know, the AC-47 "Spooky" was the first aircraft to systematically use the pylon turn in combat - in 1964, long after the B-17 had been retired.  

Strafing runs, meanwhile, were the domain of smaller aircraft. On the Allied side, the B-25 Mitchell and the De Havilland Mosquito were popular in CAS, usually bearing a heavy gun (75mm in the Mitchell, 57mm in the Mosquito) and as many machine guns as would fit. The B-17 was larger and clumsier than these craft - a large drawback when you were quite literally skimming the treetops, and, because of its limited frontal area, couldn't carry that many more guns than smaller, nimbler airplanes. The larger/more numerous guns just weren't worth the tradeoff in agility.  

TLDR; Not in any systematic fashion. The high-altitude bomber just couldn't compete with smaller airplanes.