to clear the cannon feed motors and the lower wings no longer had the gun-bay heating vents outboard of the gun-bays. The first series of Spitfire Mk IXs retained the bay doors first used on Spitfire VCs; these incorporated large blisters to clear the feed motors of two Hispano cannons. All later Spitfires had smaller, more streamlined blisters. To provide room for the belt feed system of the cannon, the inner machine gun bays were moved outboard to between ribs 13 and 14. 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. Although many earlier Spitfires had been modified to carry bomb racks for bombs the first Spitfires to be built as fighter-bombers (Modification 1209) were those of the Second Tactical Air Force. The first Mk IX Spitfires used as fighter-bombers began in June 1944. D Type 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. To avoid the expansion of fuel in hot weather damaging the wing, pressure relief valves, with small external vent pipes, were fitted near the wing tips. E type Structurally identical to the C wing. The outer machine gun ports were eliminated; although the outer machine-gun bays were retained, their access doors were devoid of empty cartridge case ports and cartridge deflectors. The new wing allowed for a four cannon armament and the inner bays could carry heavy machine guns. There were thus two possible weapon fits: either 2 × .50 cal Browning M2 machine guns with 250 rpg in the inner bays and 2 × 20 mm Hispano Mk II cannon with 120 rpg in the outer bays or 4 × 20 mm Hispano cannon with 120 rpg The .303 machine guns mounted in the outer wings were no longer fitted, because rifle calibre bullets were ineffective against heavily armoured aircraft. (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). The 20 mm Hispano cannon were moved outboard and