As we wrote, a new complete redesign was not desirable for Spitfire because it disrupted the production lines, so in February 1941 No. 92 (East India) Squadron of the Royal Air Force (RAF) became the first one to receive examples of the Mk. V which was, at first, considered as an interim fighter. This was followed by No. 91 (Nigeria) Squadron in March. Those machines were part of the initial production order for 1.000 Mk. Vs.
The Mk. V was an Mk. I/II airframe strengthened with longerons to fit a Merlin 45 engine rated at 1470 hp and by June production of the Mk. Va terminated in favour of the Mk. Vb which featured two 20mm cannons plus four 0.303 in machine guns in the wings.
Early examples of the Mk. Vb had the same span of the Mk. I/II, although, in order to improve performance at low altitudes, clipped wings were introduced later. The number of small improvements that the Mk. Vb got was very high throughout its manufacture lines and at service levels. One of the most significant was a jettisonable cockpit canopy with more bulbous edges, replacement of metal-covered ailerons and the replacement of flame-damping exhaust stubs.
By September 1941 a total of twenty-seven RAF squadron were operating the Mk. Vb, while very operated the Va. In December those numbers rose to forty-six.
The Spitfire Mk. III was a prototype variant, of which a single example was completed, N3297. It was the first significant redesign of the type and differed from previous marks by having clipped wings, a strengthened fuselage, a retractable tailwheel, an internal bullet-proof windshield and 88 lbs of armour plating. Two additional inches more forward rake was added to the strengthened undercarriage and the wheels were fully enclosed by doors when retracted.
Priority for the Hawker Hurricane Mk. II was given, so the Spitfire Mk. III wasn't put into production and was employed as a test-bed for different engines (among them the Merlin 60 and 61 - the same one that would eventually power the Mk. IX) and armaments until September 1944.
The Spitfire F.IV was the prototype for the 'F' sub-mark which was powered by the Roll-Royce Griffon engine. Its inception dates back to 4th December 1939 when Supermarine design staff printed a brochure where they proposed the idea of converting the Spitfire to the Rolls-Royce Griffon engine, expecting many performance improvements. However, the problematic development of the Griffon engine meant that the prototype of this machine would not be ready until late 1941.
The Griffon IIB which the Mk. IV was fitted with was a single-stage supercharged engine of 1735 hp. This required stronger longerons to cope with the weight of the engine, as well as bigger radiator and oil cooler, although it kept the asymmetric under-wing radiator layout of the single stage Merlin engine. The lower thrust line and larger capacity of the Griffon engine required a complete redesign of the engine cowling, with more prominent blisters over the cylinder heads and a third tear-drop shaped blister on the upper forward cowling to clear the magneto, and a deeper curve down to the spinner, which was much longer than previous marks. A four-bladed Rotol propeller was employed. This propeller catch a lot of pilots out because it rotated in the opposite direction to that of the Merlin: to the left from the pilot's perspective, instead of to the right, meaning that the sliptstream swung the Spitfire to the right on the ground, requiring the rudder to be turned to the left during take-off.
The Mk. IV DP845 flew for the first time on 27th November 1941 and was equipped with the full-span C type wing. It featured a small tail unit and retractable tailwheel. It had also external bracket hinges under the wings, denoting the installation of braking flaps, which were soon removed and the F.IV was soon also fitted with a mock-up six-cannon armament, three per wing. The aircraft was renamed to 'Mk. XX' to prevent confusion with a PR. Type and, then it became the Mk. XII.
Sources:
1st Signal Squadron - Aircraft In action 39 - Supermarine Spitfire in Action
2nd https://en.wikipedia.org/wiki/Supermarine_Spitfire_(Griffon-powered_variants)
3rd https://en.wikipedia.org/wiki/Supermarine_Spitfire_(early_Merlin-powered_variants)
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