Specification F.37/35 did not stipulate use of two engines but it did call for armament of four 20mm cannon. To achieve the requirements of the specification, Westland decided on a twin-engin layout. The Whirlwind was a fine and pleasant and in it's slender nose was an unprecedented punch.
The achillis heel of the Whirlwind was it's weak and unreliable engines. Merlins with handed propellers would have turned it into a real world-beater. With Peregrines it was useless, and only 112 out of an initial order for 200 machines was completed before production was stopped in December 1941.
Source: Fighting Aircraft of World War II
Salamander Books
