Up until 1941, early versions of the Spitfire found themselves relatively evenly matched against the Bf 109. Then, the Fw 190A came on the scene and significantly outclassed the Spitfire V. This precipitated a crisis for the U.K. If the situation remained unchanged, the Germans would gain air superiority. In fact, losses were mounting to the point that Spitfire Mk. Vs were held back from nearly all daylight RAF operations over mainland Europe.
The response featured the development of Spitfires that would be superior to the Fw 190. These new and improved variants such as the Mk. VII and Mk. VIII centered around the Merlin 60 or 70 series powerplants. These engines, along with the follow-on Merlin 63s that went into production in 1943, provided immensely improved speed and climb performance, especially between 20,000 and 40,000 feet which was exactly where German opponents had been killing Mk. Vs.
The first of the new Spitfires ready for combat was the Mk. IX. The need for a stopgap Spitfire was urgent. By late February 1942, a Merlin 63 had been adapted to the standard Mk. V airframe and the first prototype Mk. IX took to the air. The type was rushed into full-rate production by June. Soon after, they began to directly replace the Mk. V. By this time, the majority of the Spitfires from the Mk VIII on used only three basic wing types; C, D, and E. The C-type wing was known as the “universal wing” seen on most Spits after mid-1942. This standardized wing design was simplified for faster manufacture and could be fitted with various armament options.
The introduction of the Mk. IX allowed momentum in the air war to shift back towards the RAF allowing them to resume offensive Spitfire operations over Europe. Its first kill came on 30 July 1942, downing (in a telling fashion) an Fw 190. Other Mk. IX distinctions include the highest-ever interception of WWII with a Mk. IX shooting down a Ju 86R at over 43,000 feet. In late October 1944, the Mk. IX was the first allied aircraft to down an Me 262. By the end of the war, more than 5,600 Mk. IXs had rolled off the production line and many continued their service with the RAF and smaller air forces well into the early jet age.