The appearance of the Focke Wulf Fw 190 over the Western Front in September 1941 caused considerable headache, even panic, in RAF Fighter Command as it became painfully clear, that the Spitfire V was no match for the new Luftwaffe fighter. Although the Spitfire VIII was already under development, production difficulties had led to a delay of entry into operational service for a considerable time. The balance of air power had shifted firmly against the RAF and had effectively ended almost all RAF operations over occupied Europe.

The aircraft that in potential could deal with the Fw 190 menace were the pressurised Mark VII and the unpressurised Mark VIII. Both marks were to be powered by the Rolls Royce Merlin 61, but were quite some time away from being ready for production as the new engine required strengthening of the airframe.

At about the same time it was suggested that the new Merlin 61 engine should be installed in the Mark V airframe as a short term solution to restore the RAF's air supremacy.
Although initially envisaged as just an interim type, the new aircraft, designated the Type 361, the Spitfire IX, would eventually become the second most numerous variant of this legendary fighter. The exact number of Mark IXs is open to interpretation: production numbers range from 5,440 to 5,663.


Before the new high-altitude Mark IX went into production, a new German threat in the form of nuisance raids by high flying Ju 86 bombers had started in August 1942. At that time, these aircraft were immune to RAF interception. As a reaction to these raids a number of Mark IXs were stripped of all non-essential equipment to engage the raiders, but they still were unable to intercept the bombers. This only succeeded after two Mark IX Spitfires of the Special Service Flight at RAF Northolt had been modified. With all armour removed, these aeroplanes had been fitted with lightweight Jablo propellers. Also the four Brownings were stripped from the aircraft. On 12 September 1942, the special Spitfires reached 45,000 feet: the Ju 86 was attacked and damaged. Soon after these German raids came to an end.
The need of a high flying Spitfire remained and so one of the development aircraft, AB505, had its Merlin 61 replaced by a Merlin 70, the standard Merlin for the high-altitude variant. The armament of this new fighter consisted of just two 20mm Hispano cannon.


Engine for the HF.IXc:
The two-speed, two-stage supercharged Merlin 70 (1,710hp/1,275kW). The Merlins superchargers were designed to engage at higher altitudes.