The next version to enter mass production was the Spitfire Mk.IX. The main improvement of this version being the Merlin 61 two-staged two-speed supercharged engine allowing for high altitude operations to be commenced. The engine had been tested in the lone Mk.III Spitfire for use in the Wellington and found to be quite functional at high and low altitude. The Mk.IX was pushed forward as an interim aircraft whilst the Mk.VII and Mk.VIII were being developed, as a direct approach to counter the Fw190 which had outclassed the Spitfire Mk.V. Although using the Mk.V basic airframe with very little improvements, the Mk.IX was superior in performance beyond expectation due to the new Merlin 62 engine. All trials and tests were so positive that the type was approved as the next major production model as the Mk.VII and Mk.VIII would not be available until early 1943.