The Mk XVIII was designed as an anti-Submarine weapon. The Germans had responded to the increased threat from Allied aircraft by arming their U-boats with anti-aircraft guns. U-boats were then able to stay on the surface and fight it out with attacking aircraft.
The solution tested in the Mosquito FB Mk XVIII was to fit the aircraft with a massive 57mm Molins anti-shipping gun. Tests using the fuselage of a crashed Mosquito FB MK VI and a six pounder field artillery gun proved that the idea was feasible.
The prototype Mk XVIII flew in mid 1943. After air firing tests in June 1943, eighteen of FB Mk VIs were converted to carry the big six pounder in place of the 20mm cannon. The .303in machine guns were retained to help the pilot aim the big gun, which only carried 25 shells. The big gun earned the Mk XVIII the nickname “Tsetse”.
The main problem with the 57mm gun was that it required a long steady attack run. Starting from an altitude of 5000ft, the aircraft would have to dive at 30 degrees until it was within 1800 yards of its target, leaving the Mosquito rather vulnerable.
The first unit to use the Mk XVIII was No. 248 Squadron, which added it to their Beaufighters. Operations began on 24 October 1943. The first U-boat kill came on 25 March 1944, when U-976 was sunk in the Bay of Biscay by two Mk XVIIIs.
The Mk XVIII was also used by Nos. 235 and 254 squadrons, and saw action against surface ships in the build-up to D-Day. However, the heavy cannon was generally felt to be less effective than normal rockets, which could be carried by the standard FB Mk VI.
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