The debut of the formidable Focke-Wulf Fw 190 in late 1941 had caused problems for RAF fighter squadrons flying the latest Spitfire Mk Vb.

In a second stream of development Supermarine was working on an improved, reinforced, Spitfire airframe which incorporated several new features and was designed for the Merlin 60 and 70 series engines.

Stronger undercarriage legs were raked 2 inches (5.08 cm) forward, making the Spitfire more stable on the ground and reducing the likelihood of the aircraft tipping onto its nose.

[4] Several versions of the Spitfire, starting with the HF Mk VIIs had extra 13 gallon integral fuel tanks added to the wing leading edges between the wing-root and the inboard cannon bay.

[7] These were made for photo-reconnaissance Spitfires, including the PR Mk X and XI; no armament was fitted and the D-shaped leading edges of the wings, ahead of the main spar, were converted into integral fuel tanks, each carrying 66 gallons.

(These outer guns had always been the less effective part of a Spitfire's armament: their distance from the centreline made them hard to harmonise and in turning engagements, wing flexing meant that the rounds were even more widely scattered).

This armament was standard for all Spitfire Mk IXs and XVIs used by the 2nd Tactical Air Force as fighters and fighter-bombers from shortly after D-Day.

The Mark numbers did not necessarily indicate a chronological order, the Mk IX was a stopgap brought into production before the Mks VII and VIII.

Early Mk IXs had a teardrop shaped blister (a bulge) for a Coffman engine starter[nb 1]on the lower starboard side cowling, just behind the propeller.

The Mk VII used a Marshall manufactured compressor for pressurising the cockpit; this was mounted on the right of the engine and drew its air through a long intake under the starboard exhaust stubs.

[23] The HF Mk had superb high-altitude performance, with a service ceiling of 45,100 feet (13,700 m); French ace Pierre Clostermann recalls in his book, The Big Show, the successful interception of a long-range reconnaissance Messerschmitt Bf 109G-6/R3 by a Mk VII 'Strato Spitfire' of 602 Squadron at 40,000 feet (12,000 m) over the British Home Fleet's base at Scapa Flow in early 1944.

The performance increase was described by Jeffrey Quill as a "quantum leap" over that of the Mk Vb and another Spitfire airframe, R6700 was modified to take the new engine.

Although design work on the Mk VII and VIII series was under way, these would take over a year to get into production and a counter to the Fw 190 was urgently needed.

These could be identified by the Type C wing with the large double blisters over the inner cannon bays and the identification light on the fuselage spine, behind the aerial mast.

Two Spitfire Mk IXs converted from Vcs were stripped of armour, the .303 Brownings and other equipment and repainted in a lightweight PRU blue finish.

Serial listings show that the HF Mk IX was produced in relatively limited numbers when they were required, with priority being given to versions rated for low and medium altitudes[nb 6] Maximum power of the Merlin 70 was 1,710 hp (1,280 kW) at 11,000 ft (1,275 kW at 3,353 m): maximum speed of the Spitfire HF Mk IX was 405 mph (652 km/h) at 25,400 feet (7,700 m) at an all-up weight of 7,320 lb (3,320 kg).

As well as incorporating camera equipment, a wrap-around PR type windscreen was fitted and a larger oil tank was installed under the nose.

16 Squadron, which was a unit of the 2nd TAF, used several FR Mk IXs (painted a pale, "Camoutint" Pink, which provided excellent camouflage under cloud cover) to photograph the Arnhem area before and during Operation Market Garden.

In 1951, a further six TR 9 trainers were converted from the standard Mk IX to train pilots for the Irish Air Corps (IAC) Seafire fleet.

The Spitfires provided transition training that included gunnery practice since the type was equipped with two .303 Browning machine guns, one in each outer wing bay.

It had the pressurised Mk VII cockpit, with the Lobelle sliding canopy, and retained the fighter style windscreen with the bullet-proof glass panel.

[48] Physically the Mk XIs had a deeper nose fairing to accommodate a larger 14.5 gal oil tank and used the unarmoured, wrap-around PRU windscreen.

Air Vice Marshal John Slessor, head of Coastal Command pointed out that the Spitfire was smaller than the Mosquito, used half the number of Merlin engines and was faster, more manoeuvrable and quieter and, therefore production should be increased, not reduced.

Almost all production Mk XVIs had clipped wings for low altitude work and were fitted with the rear fuselage fuel tanks with a combined capacity of 75 gal.

[38] Because of a slightly taller intercooler and rearranged accessories on the Packard Merlins a new, bulged upper cowling was introduced and also appeared on late production IXs.

Tank tests were carried out at Farnborough, using a 1/7 scale model, it was found that the concept was basically sound, although the vertical tail surfaces would need to be enlarged to counterbalance the side area of the floats.

At the time it was thought that the floatplanes could operate from concealed bases in the Dodecanese Islands, disrupting supply lines to German outposts in the area which relied on resupply by transport aircraft.

This scheme came to naught when a large number of German troops, backed by the Luftwaffe, took over the British held islands of Kos and Leros.

[56] Five aircraft were converted: Supermarine 305: Designed to the same F.9/35 specification which led to the Boulton Paul Defiant turret fighter (and the rejected Hawker Hotspur).

The basic F.37/34 (as Spitfire was still then known) wing and undercarriage were mated to a modified fuselage which provided room for a gunner and a remote control four-gun turret (originally armed with .303 Brownings, later with Lewis light machine guns.)