In 1939, the Hawker design staff had studied alternative powerplants for the Hurricane. However the modifications required for the fitting of the Rolls Royce Griffon or the Bristol Hercules would most certainly lead to a reduced production rate at a time where a steady flow of aircraft into operational RAF-service was absolutely essential. In February 1940, the new two-stage supercharged Merlin XX, initially producing 1,185 hp (868 kW), was proposed. This engine had been designed for mass-production and became available only weeks later. On 11 June 1940 the first Hurricane with the Merlin XX, P3269, made the first flight. In August 1940 the Merlin XX achieved full production status. The first Hurricanes with the new Merlin XX were designated Mk.IIA Series 1, and differed only slightly from the earlier Mk.I. The aircraft retained the Mk.I wing with eight 0.303in (7,7mm) Browning machine guns.

Enter the Hurricane Mk.IIB...
Almost from the start of the Hurricane's operational service, the improvement of firepower had been under consideration. The eight Bowning machine guns had a high rate and density of fire, but it also became apparent, that the light calibre 0.303 inch (7,7mm) bullets were increasingly becoming ineffective.

Apart from a proposal for fitting four cannon in the Hurricane as early as 1936 and a trial with two 20mm Oerlikon cannons mounted under the wing of Hurricane I L1750 in 1939, Hawker submitted a scheme to install four additonal Brownings in the wings. Mark II Hurricanes armed with the twelve guns were designated Mk.IIB.

In the fighter-bomber role, with two underwing 250lb or 500lb bombs, only eight guns would be installed. Alternatives to the bombs initially included six (later eight) rocket projectiles, two fixed fuel tanks of 44 gallons each or two drop tanks. Of the latter two versions were available: with 45 or 90 gallons capacity each. The rocket projectiles had a 60lb (27.2kg) warhead and four stabilizing fins at the rear of the rocket fuselage containing the propellant. These weapons were first fired in anger by Hurricanes of Nos. 137 and 164 Squadrons against the Hansweert locks complex in the Netherlands on 2 September 1943.