The Type 300 was considerably cleaned-up, with progressive refinements including retractable undercarriage, an enclosed cockpit, oxygen-breathing apparatus, and smaller and thinner wings.

The distinctive elliptical wing was developed during this phase, enabling it to accommodate the armament and undercarriage in the thinnest possible cross-section, and thus helping the aircraft achieve a high maximum speed.

[3][4][5] The outer wing section was too thin to fully enclose the additional guns, so small underside blister fairings would be added to production machines to make room for the mechanism.

Meanwhile the PV-XII had changed its coolant from water to ethylene glycol, allowing the clumsy evaporative cooling system to be abandoned and replaced by a ducted radiator which had recently been developed by Frederick Meredith at the Royal Aircraft Establishment, Farnborough and actually provided a small amount of jet thrust.

The engine fitted for initial trials was a prototype Merlin C 990 hp (738 kW), with six stub exhaust ports just protruding from each side, driving an Aero-Products "Watts" two-bladed, wooden fixed-pitch propeller.

Workmen experienced on the car applied a coat of filler to cover all the rivets, panel joints and other surface blemishes, and rubbed it down to a smooth finish.

This is often misunderstood to mean that the Spitfire was flawless, but in fact Summers just wanted to talk the flight over with Mitchell and the design team before anything, especially the control settings, was altered.

Returning to Eastleigh, the rudder balance, air intake and engine cowlings were modified, the aircraft given the blue paint scheme described above and the undercarriage doors fitted.

A new propeller was designed and made, on 15 May taking the top speed to 348 miles per hour (560 km/h), at last visibly outclassing the Hurricane and earning it the reputation as the fastest military aircraft in the world.

[26] Specification F.16/36, issued to accompany the order, incorporated so many improvements that what was effectively a full new set of design drawings was needed for the production version.

A couple of days later Summers took it to Hatfield for the SBAC show where it was the star exhibit, giving a display of aerobatics and attracting intense interest from media and industry alike.

[31] The engine was also changed for an uprated Merlin F. Trials continued with split peas glued onto the airframe to simulate dome-headed rivets, which were less costly and time-consuming, but also caused greater drag, than flush countersunk ones.

[32] Following an engine failure due to low oil pressure and consequent wheels-up forced landing at the hands of Sam McKenna on 22 March 1937, a prototype triple ejector exhaust was fitted.

Besides a number of other small improvements, the aircraft was repainted in standard RAF camouflage of Dark Earth/Dark Green on the upper surfaces with a silver dope finish underneath.

On 19 September, it was found that the new exhausts developed 70 pounds of thrust, equivalent to about 70 hp at 300 mph and pushing the maximum speed up to 360 miles per hour (580 km/h).

[33] Development work continued, especially on the engine exhausts and gun heating system, with the occasional landing mishap, until in July 1938 K5054 was sent back to Farnborough.

The fuselage broke up and White suffered fatal neck injuries from the anchor point to the Sutton safety harness, dying the next day.

[38] The aerobatic sequences featured in the last 15 minutes of the film were flown by Jeffrey Quill, an original test pilot on K5054, in early November 1941 flying a Spitfire Mk II mocked up to represent the prototype.

Former Supermarine test pilot Jeffrey Quill determined in 1983 to build an exact full-scale replica of K5054 for permanent public display as a memorial to its designer, R.J. Mitchell.

Together with Mitchell's son Gordon, members of the original Supermarine design team and The Spitfire Society, they commissioned Aerofab Restorations of Andover to create the facsimile.

[43][44] A sub-scale model of K5054 forms the main centrepiece of a Spitfire memorial sculpture on the roundabout at the entrance to Southampton Airport which, as Eastleigh Aerodrome, was the aircraft's initial home.

It is built in accurate proportion around a three-quarter scale set of outer wing panels and some fuselage structure from the Australian Supermarine Spitfire Mk 25 homebuild kit.