The B-17G, known a Fortress III in RAF service, wouldn't quite fit the GB, even if it had been used by CC. The CC aircraft, although tasked with recce duties, were also ASW/anti-shipping aircraft.
However, it could fit into the heavy hitters GB, or, if the rules were slightly bent for GB17, as a recce aircraft in a way. The Fortress III was used by RAF 100 Group, in the ECM role, and also as a 'listening post', intercepting enemy radio traffic. Would need quite a bit of work to get the right equipment fit for external appearance, but could be done.