The Mk I and II versions demonstrated their formidable performance during the "Battle of Britain", and by early 1941, the majority of fighter squadrons were furnished with this fighter. Improvements were suggested by the pilots and every effort was made to maintain its performance advantages over its opponents. Early improvements resulted in the Mk. V series of Spitfires which became the most widely produced version of all Mk's. The main improvement was the furnishing of three wing armament versions, and the use of the more powerful Merlin type 45 engine, and the "fishtail" flame damping exhaust stubs. The most numerous Mk.V was the "b" winged version with a mixed armament of two 7.7mm machine guns and one 20mm cannon in each wing.