era, British turrets were largely self-contained units, manufactured by Boulton Paul Aircraft and Nash & Thompson. The same model of turret might be fitted to several different aircraft types. Some models included gun-laying radar that could lead the target and compensate for bullet drop. As almost a 1930s "updated" adaptation of the earlier Bristol F.2's concept, the UK introduced the concept of the "turret fighter", with aeroplanes such as the Boulton Paul Defiant and Blackburn Roc where the armament (four 0.303 inch) machineguns was in a turret mounted behind the pilot, rather than in fixed positions in the wings. The Defiant and Roc possessed no fixed, forward-firing automatic ordnance; the World War I-era Bristol F.2 was designed with one synchronized Vickers machine gun firing forward on a fuselage mount. The concept came at a time when the standard armament of a fighter was only two machine guns and in the face of heavily armed bombers operating in formation, it was thought that a group of turret fighters would be able to concentrate their fire flexibly on the bombers; making beam, stern and rising attacks practicable. Although the idea had some merits in attacking unescorted bombers the weight and drag penalty of the turret (and gunner) put them at a disadvantage when Germany was able to escort its bombers with fighters from bases in Northern France. By this point British fighters were flying with eight machine guns which concentrated firepower for use in single fleeting attacks of fighters against bombers. Attempts to put this heavier armament, such as multiple 20 mm cannon in low profile aerodynamic turrets were explored by the British but were not successful, this class of weapons and heavier armament (up to and including artillery pieces as in the 1,420 examples produced of the American B-25G and B-25H Mitchell medium bombers, and the experimental 'Tsetse' variant of the de Havilland Mosquito) being exclusively fuselage or underwing-mounted and thus aimed by pointing the aircraft as a whole. Not all turret designs put the gunner in the turret along with the armament: US and German-designed aircraft both featured remote-controlled turrets. In the US, the large, purpose-built Northrop P-61 Black Widow night fighter was produced with a remotely operated dorsal turret that had a wide range of fire though in practice it was generally fired directly forward under control of the pilot. For the last Douglas-built production blocks of the B-17F