The FB Mk VI, which first flew on 1 June 1942, was powered by two 1,460 hp (1,088 kW) Merlin 21s or 1,635 hp (1,218 kW) Merlin 25s, and introduced a re-stressed and reinforced "basic" wing structure capable of carrying single 250 lb (115 kg) or 500 lb (230 kg) bombs on racks housed in streamlined fairings under each wing, or up to eight RP-3 25lb or 60 lb rockets. In addition fuel lines were added to the wings to enable single 50 gal (227l) or 100 gal (455l) drop tanks to be carried under each wing. The usual fixed armament was four 20mm Hispano Mk.II cannon and four .303 (7.7 mm) Browning machine guns, while two 250 lb (115 kg) or 500 lb (230 kg) bombs could be carried in the bomb bay. Unlike the F Mk II, the ventral bay doors were split into two pairs, with the forward pair being used to access the cannon, while the rear pair acted as bomb bay doors. The maximum fuel load capable of being carried was 719.5 gal (3,271l) distributed between 453 gal (2,059l) internal fuel tanks, plus two overload tanks, each of 66.5 gal (302l) capacity, which could be fitted in the bomb bay, and two 100 gal (455l) drop tanks. All-out level speed is often given as 368 mph (592 km/h), however this speed applies to aircraft fitted with saxophone exhausts. The test aircraft (HJ679) fitted with stub exhausts was found to be performing below expectations. It was returned to de Havilland at Hatfield where it was serviced. Its top speed was then tested and found to be 384 mph (618 km/h), in line with expectations. 2,298 FB Mk VIs were built, nearly one-third of Mosquito production. 18 built by Airspeed Ltd were eventually modified to become FB.XVIIIs. Two were converted to TR.33 carrier-borne, maritime strike prototypes.The FB Mk VI proved capable of holding its own with single-engine fighter aircraft. For example, on 15 January 1945 Mosquito FB Mk VIs of 143 Squadron were engaged by 30 Focke-Wulf Fw 190s from Jagdgeschwader 5: five Mosquitos were lost but five Fw 190s were shot down in return, and an armed trawler and two merchant ships were sunk.