Supermarine Spitfire Mk.III

Spitfire III 2

The legend of the Supermarine Spitfire was forged in the skies over Britain during the summer of 1940. The graceful looking aircraft made it easy for the British media spindoctors to turn R.J. Mitchell’s design in to an unbeatable weapon of war as far as the public image was concerned but this hid the truth that already its shortcomings were becoming obvious even before the Battle of Britain and the early Spitfire Mk.I and the slightly improved Mk.II would need replacing by 1941.

The Supermarine engineers returned to the drawing board and looked at almost every aspect of the aircraft taking in to account the lessons learned from the early experiences in service. Assigned the in-house designation of Supermarine Type 330 the Spitfire Mk.III would need to be faster than its predecessors in order to allow it to keep up with the latest German fighters and to achieve that the Rolls-Royce RM 2SM engine (later redesignated Merlin XX) was chosen to power the new type. This provided an additional 215hp over the Merlin XII as fitted to the Spitfire Mk.II and was the first Merlin engine to feature a two-speed supercharger.

Not satisfied with simply putting a more powerful engine in the aircraft the designers at Supermarine incorporated a number of aerodynamic improvements including the fitting of doors over the wheels that closed when they were retracted in to the wing thus preventing the space around the wheel from producing drag. Additionally the tail wheel was made to retract in to the rear fuselage and the armoured windscreen was moved inside the cockpit both of which improved aerodynamic efficiency.

Spitfire IIIPerhaps the most noticeable change was the cropping of the Spitfire’s signature pointed wingtips making them come to a flat straight line. This reduced the Spitfire III’s wing span from 36ft 10in in the Mk.I/II to 30ft 6in which improved roll rate at lower levels, something for which the Spitfire was found to be lacking compared to its rivals, but high altitude performance dropped off slightly. The reduced forward cross section of the aircraft with cropped wings also helped improve acceleration by reducing drag. The wing was essentially a modified “c” wing which allowed the aircraft to be fitted with different weapon options (see below). A less obvious change to the aircraft was a 7in fuselage extension to fit the engine and balance it out.

All these efforts resulted in the first Spitfire III being able to attain a maximum speed of 400mph at 21,000ft, almost 50mph faster than the previous Spitfires. This offered a big advantage in the air compared to some of the aircraft’s rivals in late 1940;

  • Messerschmitt Bf109E – 336mph at 19,685ft
  • Macchi C.202 – 372mph at 18,370ft

The first Spitfire Mk.III was N3297 and the aircraft first flew on March 16th 1940. Testing of the aircraft continued despite the constraints of the Battle of Britain that was raging overhead and the RAF was sufficiently interested in the aircraft to eventually put in place a production order for over 1,000 aircraft with deliveries to begin in early 1941. However the Merlin XX was already earmarked for use in the Hawker Hurricane Mk.II project which was seen as a higher priority largely thanks to the fact that the Hurricane needed the uprated engine more than the Spitfire to make up for its shortfalls in combat with the Bf109. The Spitfire Mk.III was not abandoned but merely put on hold and N3297 continued testing in order to perfect the design.

As 1941 came it was becoming increasingly clear that the window for the aircraft to enjoy its advantages to the fullest was rapidly closing thanks to the Merlin XXs going to the Hurricane. Also Rolls-Royce was already developing their Merlin 45 series of engines which would ultimately power the Spitifre Mk.V which was ordered in to production in late 1940 with the first examples arriving just a few months later. Despite this the Spitfire Mk.III prototype was providing a lot of useful data to Supermarine allowing them to fine-tune the airframe changes and incorporate them in to production Spitfires. A second aircraft, Spitfire Mk.V W3237, was configured to an almost identical standard (minus the retractable tailwheel) to continue Mk.III testing while N3297 was fitted with a standard “A” wing and delivered to Rolls-Royce for engine testing eventually being fitted with the first Merlin 61 engine that would be used in the Spitfire Mk.VIII and IX. Even with a second prototype the Spitfire Mk.III project was now cancelled altogether but W3237 continued testing until 1944 to support development work on newer model Spitfires.

While the Spitfire Mk.III was a non-starter the fruits of the testing carried out by the two prototypes can be seen in many of the improvements made to the later Spitfire marks such as cropped wings and doors over the landing gear. The Merlin XX did find its way in to around 50 Spitfire II aircraft converted for use in the air-sea rescue role.


ENGINE: 1 x 1,390hp Rolls-Royce Merlin XX inline engine

MAX SPEED: 400mph at 21,000ft

WINGSPAN: 30ft 6in

LENGTH: 30ft 4in

HEIGHT: 9ft 10in

WING AREA: 220ft2


  • 8x .303 (7.7mm) Browning machine guns
  • 2x 20mm Hispano cannons / 4x .303 (7.7) browning machine guns
  • 4x 20mm Hispano cannons

12 responses to “Supermarine Spitfire Mk.III

  1. Pingback: Defence of the Realm – Technology | Defence of the Realm

  2. The whole range of spitfire marks is a challenging topic and one, I for one, have never been able to get my head fully round. I have a number of books and ‘family’ charts but it still remains a mystery. If I know you Tony, it will all be clear by the end.

    Liked by 1 person

  3. A few points and links which maybe of interest.

    Spitfire marks became chaotic as what went in production when was directed by needs.
    an attempt to standardise Mk numbers was made in ‘teens’ but even that is not to logical.
    it’s like the English language, idiosyncratic and illogical, you just have to learn it, just don’t expect it to make logical ‘sense’

    Alfred Price ‘The Spitfire Story’ is the best book I have seen for sorting what is what,

    Does not cover Seafires though.

    PR Spitfire can be especially confusing, as can working out what Seafire corresponds to what Spitfire!

    Regarding the Spitfire III.
    The cropped wing was not a ‘C’ wing, that did not as yet exist, at the time only some basic work had been done on fitting cannon as well.
    AFAIK, it was a modified A wing, but I’d have to recheck my books to be sure.

    Regarding the crop, this is not the same as the later clipped wing, which was a short tip on a standard wing. That wing could also have standard or extended high altitude tips, as fitted to Mk VI and VII, and some early VIII’s.
    this was changeable within a few hours, so could be changed in service.

    The original Mk III wing is shorter than a standard wing with clipped tips.

    the types of Spitfire wing are discussed here

    which has other useful articles on a complex subject!

    The Spitfire III has been discussed on Britmodeller a couple of times

    while some of the discussion is about what base model to use, there is much discussion about what was changed and when by some very knowledgeable folk.

    for the truly Spitfire obsessed there is the Shacklady ‘Spitfire – the History’ which is a 600 page monster

    but a lot less user friendly than the Spitfire Story.

    hope of use


  4. from here,
    but taken from ‘Spitfire – The History’

    N3297 was a Mk I (from the production line) modified to Mk III:
    – Installation of the Merlin XX necessitating a modification to the cowling (slanted firewall) and adding 4 inches in overall length.
    – The Merlin XX required a larger oil cooler (eventually used on the Mk V).
    – The radiator housing was of 2 different types during its career, slightly different and both very deep (similar to later Griffon in size).
    – The windscreen/canopy was developed over time, eventually resulting in internal armor glass and bulged canopy similar to the later Mk V and IX.
    – Tail wheel was retractable (as later installed on the VII/VIII, etc)
    – Wings (this is where it gets real interesting)
    + the original wing on the Mk III was a modified A wing. The specifications are identical to the Mk I wing except in span (and hence area, wingloading, etc). Standard span is 37 feet, 1 inch. On page 127 it states 3.5 feet were removed from each tip resulting in a span of 30 feet, 1 inch with a w/a of 220 sq ft. note: this is shorter than the later clipped wing span of 32 feet, 7 inches with a w/a of 231 sq ft.
    On page 128 it states, “Wing surface. The wing area is slightly reduced without interfering with the main structural members, except at the tip where the area is removed.”
    + later (Sep 1940) the wing was extended to standard length and even later (Mar 1941) it was replaced with what eventually became the C wing. Of note, the authors indicate Dowding did not like the clipped wings.
    + Ailerons were short span on original modified wing, illustration on page 130 gives the impression it went to the wing tip cover. Span (specs page 132) was 5.42 feet. Normal aileron span “appears” to be 5.54 feet (on page 74 this is listed as chord, but the math would indicate the the span would need to be 3.4 feet!) This 3 inches seems similar to the short span ailerons on the VII/VIII, however the area is significantly smaller on the Mk III (16.4 sq ft compared to 18 sq ft on the VII/VIII). No indication to whether ailerons were fabric or metal.

    So from a modelling perspective:
    – Early Mk III is unique in appearance.
    – Mid Mk III with A wing would look like a “long nosed” Mk Va.
    – Late Mk III with C wing would look like a “long nosed” Mk Vc.
    – Final Mk III (N3297) with Merlin 60 was the prototype MK IX with detail differences (tailwheel, radiator, oil cooler).

    W3237 was a Mk V removed from the assembly line modified to Mk III and looked more like the late Mk III N3297. W3237 later became the testbed for extended tips (Mk VI and VII) and Seafire flap settings.

    Liked by 1 person

  5. Pingback: 80 Years Of A True Legend Of The Skies | Defence of the Realm

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