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Bombardier at work.
1944
Hatfield, England

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The standard British air weapon early in WW II was the Browning 0.303 inch machine gun. It was a well proven weapon in good supply, but it lacked hitting power. At first the solution was just to add more, until the Hurricane fighter ended up with no less than 12 guns. The Americans preferred the heavier Browning 0.50 inch machine gun.

The 20 mm. Hispano cannon with explosive shells provided the extra punch that British fighters needed. It was relatively slow firing at 10 shells per second, but worked well in combination with the machine guns. Although the belt fed cannon had been prone to jamming, the British BSA company managed to copy a new feed design that solved the problem. It was called the Chatellerault, and was obtained just before the fall of France.

There were doubts about whether the wooden structure of the Mosquito could absorb the considerable recoil force of four cannon, when breakages of metal mounts were common. However, exhaustive testing proved that wooden mounts actually absorbed the force better. The Mosquito F.B. Mark XVIII eventually mounted the 3.7 inch Molins gun with no problems.