When the Focke-Wulf Fw 190 appeared over the Channel in late 1941, it outclassed the Spitfire Mk V so thoroughly that the RAF briefly suspended daylight operations. The answer was a desperate improvisation: marry the new Rolls-Royce Merlin 61—with its two-stage, two-speed supercharger—to a modified Mk Vc airframe. The first Mk IX flew on 26 February 1942 and entered service that June. Test pilot Jeffrey Quill called the performance jump a "quantum leap." It was faster, climbed higher, and finally gave the RAF a fighter that could match the "Butcher Bird" on equal terms. Over 5,600 were built, making the Mk IX the most numerous Spitfire variant—and the one that bore the brunt of the air war from 1942 onward.