The Spitfire Mk VB was the main production version of the Mark Vs. It was powered by the Merlin 45 series engine with the B wing becoming the standard. As production progressed, additional changes were incorporated which also became standard on some later Spitfires. Production started with several Mk IBs which were converted to Mk VBs and armed with two 20mm cannon with four machine guns. Its main opponent over the summer of 1941 was the Messerschmitt Bf 109F.