The Stirling Mk III was powered by 1,635-hp (1219-kW) Hercules VI or XVI engines. Apart from the minimal increase in power, the primary benefit of this powerplant was that it was substantially easier to maintain. The Mk III underwent several internal changes and was fitted with a new, more streamlined dorsal turret to replace the angular model used in the Mk I. Production of the Stirling reached its zenith, with 80 aircraft produced each month by mid-1943. The final Stirling bombers were completed in the autumn of 1944, as the delivery of Halifax and Lancaster bombers increased. Due to the Stirling’s inability to reach the operational altitude of around 20,000 ft (6100 m) that newer bombers achieved, and the bomb bay’s inability to adapt to the ever-increasing size of bombs, they began to be withdrawn from service for other tasks. The last operational Stirling sortie flown by Bomber Command was conducted by No. 149 Squadron on 8 September 1944.