The most numerous Halifax variant was the much improved B Mk III of which 2,091 were built. First appearing in 1943, the Mk III featured the Perspex nose and modified tail of the Mk II Series IA but replaced the Merlin with the more powerful Bristol Hercules XVI radial engine. Other changes included the adoption of de Havilland Hydromatic propellers and a wider wing span with rounded wing tips. With the coming of the MkIII the Halifax's performance finally matched that of the Lancaster though the latter had a larger bomb load and could take larger bombs. The B.VI Halifax's performance improved still further with a cruising speed of 265 mph and a maximum speed (in 'Full Speed' supercharger mode) of 309 mph at 19,500 ft. Halifax crews, though admittedly not unbiased, considered the MkIII Halifax to be the equal of any other bomber, including the Lancaster, and further improved versions (with more powerful Hercules engines) to be superior to all. The improvement in the Halifax MKIII's performance could be measured objectively. In 1943 4 Group's Halifax squadrons flew 11,607 sorties for a loss of 485 aircraft, a loss rate of 4.2%. Halifax MKIII production started in early Autumn 1943 and for 1944, when the MkIII constituted an increasing percentage of the Halifax force, 4 Group flew 25,454 sorties for a loss of 402 aircraft, a loss rate of 1.6%.