The Mk XVIII Mosquito was a rare variant as only 18, including the prototype HJ732, were built. The aircraft were HX902, 904, MM424, 425, NT220, 224, 225, and PZ251, 152, 300, 301, 346, 467, 468, 469 and 470. Most served with 248 Squadron and were in use until the end of the war. The exception was PZ467 which was sent to the USA for evaluation and did not see front line service. What set all these aircraft apart was that they were all fitted with the Molins 6 pounder anti-tank gun.
The Molins anti-tank gun was produced by Molins Machine Co of Peterborough at the beginning of the war. It was intended to be mounted on small armoured vehicles for use against tanks. It held 22 or 25 rounds in batches of 4 or 5 which were fed electrically to the breech. As one batch was fired the next was moved into position. It completed trials in 1942 but the Germans then introduced the Tiger tank which was impervious to 6 pounder shells.
The Hawker Hurricane IID's were fitted with 40mm Rolls Royce BF or Vickers Type S anti-tank guns but it was thought that something heavier was needed such as the Molins. As the Molins weighed 1,800lbs (816kg) this would mean a bigger, preferably twin engined, aircraft and de Havilland were approached to see if the Mosquito would do. As they had already done a feasibility study on the much heavier 3.7 inch (94mm) gun they agreed it was possible.
An old FBVI fuselage was used for static firing tests but a bolt broke so another method of attaching the gun had to be found. A new FBVI Mosquito, HJ732, was selected and modified to a Mk XVIII. The 12ft 5in (3.8mm) gun was mounted at a slightly downward angle and protruding 2 feet (600mm) from the nose. It now really looked like a Mosquito with a sting.
After further ground firing tests flying tests took place and a snag was found. If the Molins gun had a force of more than 2.5g imposed on it then it would not load the shells. Modifications were made so it was capable of sustaining both negative and positive g forces. However after firing 400 rounds the under surface of the flaps tore off. After strengthening of the flaps it was decided that only the two outer .303 Browning machine guns would be retained but with a greatly increased ammunition capacity.
Another minor problem was that the gun's breech was behind the crew and the spent shell cases were ejected inside the aircraft fuselage where they rolled about with aircraft movement. It was thought that if they were ejected externally they might hit the tailplane.
To accomodate both the Molins and the Browning machine guns a different gunsight, the MkIIIa, replaced the MkII. This had different aiming marks for the Molins and the Brownings. In addition protective armour and long range fuel tanks were fitted so the aircraft could be used against U boats.
The aircraft served with 248 Squadron which was based at Predannack in Cornwall from October 1943. They proved to be a great success and the first U boat, U-123, was sunk on November 7 in the Bay of Biscay. The Molins armour piercing shell weighing 7.1lbs (3.2kg) were tipped with hardened steel and had no problems entering a submarine's pressure hull and creating great havoc inside.
248 Squadron protected Allied shipping during the D-Day landings and then was moved north to Banff in Scotland in 1944. From there it carried out many attacks on German shipping and installations in Norway.
Specification of the Molins 6 pounder Anti-tank Gun.
Bore 2.25in (57mm)
Action Recoil
Cyclic Rate 60 rounds per minute
Muzzle Velocity 2,600ft/sec (792m/sec)
Ammo feed Molins automatic
Magazine 22 rounds (some sources say 25)
Length 12ft 5in (3.8m)
Height 38in (965mm)
Weight 1,800lbs (816kg)