Altogether more successful was the Handley Page Halifax that equipped No.35 Squadron. In all 6,176 of these bombers would be produced in 8 different versions, and it would not leave RAF service until 1952. The early versions of the Halifax suffered from similar problems to the Stirling, but the designers at Handley Page had anticipated that technical advances might be made and had included allowance.
The original Halifax MkI was able to achieve a ceiling of 18,000 feet – not much better than the Stirling – and a speed of 265mph. The MkII had the more powerful Rolls Royce Merlin 20 engines fitted, which improved performance somewhat, though the added weight of larger fuel tanks and a dorsal turret rather negated this.
The Halifax seemed fated to fly missions that needed heavy bombloads delivered over short distances or to lightly defended targets. In the summer of 1943, however, the Merlin engines were replaced by Bristol Hercules engines to create the MkIII. This version could reach 309mph and 22,000 feet with a range of 1,250 miles. The Halifax therefore took its place as a true long-range heavy bomber as had been intended. The Halifax would be produced in nine different versions, including a cargo carrier, paratroop transport and tropical conversion.
from "RAF Bomber Command at War" by Rupert Matthews.