FK185 with Vickers 40mm S gun installed in a Bristol B.16 turret in the nose
In 1943, the U-boats of the Kriegsmarine adopted a strategy of remaining surfaced and utilizing their multiple 20 and 37 mm weapons to engage attacking Allied aircraft. This approach initially proved successful, resulting in the loss of numerous Coastal Command aircraft. The issue arose because the front turrets of aircraft like Sunderlands, Wellingtons, and others were equipped with 0.303-inch guns, leaving a critical 1 km gap during the attack when the aircraft couldn’t effectively return fire. The small Brownings, sometimes only a single gun, had a maximum effective range of approximately 500 metres (1,800 feet), significantly shorter than the heavy-caliber AA guns mounted on the U-boats.
Naturally, the Air Staff became concerned, leading to meetings with various aircraft and armament companies to devise a solution for countering this new and potentially disastrous situation. Among the suggested ideas was the installation of a Vickers Type S 40 mm gun in the nose of Coastal Command aircraft. Unfortunately, the front fuselages of aircraft like Sunderland, Whitley, Hudson, and others were unsuitable for such modifications. However, the Command had several squadrons of Boeing B-17, offering a viable option. The Bristol Company received an order to design a suitable turret to mount the S gun and adapt a Fortress to accommodate it.