Actually, this may not be technically what you mean, but going on with the B-17 example, IL-2 Sturmovik is a WWII flight simulator where online multiplayer coop matches can be arranged as you described, with one player the pilot, bombardier, gunners, etc., flying alongside single-seat fighters or other multi-crew aircraft. Although the Fortress itself is not included as a flyable aircraft with cockpit textures itself, other, smaller contemporaries like the B-25, G4M1-11 'Betty', and A-20 are.