At the time of the Munich Crisis in September 1938, when the start of World War II was less than one year distant, it was unclear to the British authorities, in the government, the services and industry, whether or not production of the Hawker Hurricane and Supermarine Spitfire fighters could be accelerated sufficiently to allow all of Fighter Command’s day interceptor squadrons to be re-equipped by the time the increasingly inevitable European war began. This fact served to focus both government and industry attentions on the continued availability of strategic materials, which could become critical in the event that the UK suffered a sea blockade and isolation from her sources of minerals.
The diminutive Martin-Baker company was already engaged in the creation of an all-wooden fighter, the M.B.2, as a demonstration of what could be achieved without the large-scale use of steel and light alloys, or the Rolls-Royce Merlin engine. Another company, not hitherto successful in producing a single-seat fighter, was Miles Aircraft Ltd of Woodley, Reading, and before the end of 1938 Frederick George Miles, formerly chief designer of Phillips and Powis Aircraft (Reading) Ltd, offered the Air Ministry the design of another fighter of predominantly wooden construction. This was the M.20 to be powered by the Rolls-Royce Peregrine liquid-cooled V-12 engine and owing much of its layout to the successful Miles Master advanced trainer. The Air Ministry did not accept this initial M.20, largely because production of the Peregrine was reserved for the Westland Whirlwind twin-engined and was, in any case, being reduced so as not to compromise delivery of the all-important Merlin engine. Miles persisted with his contention that a fighter could be produced not only in wood, but employing numerous components common to an aeroplane type which was already in production. After Lord Beaverbrook became Minister of Aircraft Production in May 1940, Miles’s proposal won qualified approval, Specification F.19/40 being raised to define the type and a single prototype being ordered.
Redesign of the M.20 was undertaken by Walter G. Capley, who produced a low-wing cantilever monoplane design as the the M.20/2. This eliminated the Master’s cranked wing; introduced the powerplant of one Merlin XX, rated at 1,260 hp (939.5 kW) for take-off and driving a three-blade Rotol propeller of the constant-speed type, in the form of the complete ‘power egg’ installation developed for the Bristol Beaufighter Mk II twin-engined warplane; and added fixed tailwheel landing gear with wide-track cantilever main units with large fairings and wheel spats. The wing was notably thick in section, and carried eight 0.303-in (7.7-mm) Browning Mk II fixed forward-firing machine guns with provision for a further four weapons of the same type to be fitted if required. The structure was of spruce and plywood throughout, except at the wing tips and wing root fairings, which were of light alloy. Unique at the time was the single-piece moulded Perspex cockpit canopy, which was arranged to slide fore and aft to provide access to the cockpit without interrupting the fuselage structure, and also bestowed field of vision superior to those of any in-service fighter at the time.
The M.20/2 recorded its maiden flight on 15 September 1940, only 65 days after design of the aeroplane had been launched. Despite its cumbersome landing gear, the M.20/2 posted a maximum speed of 304 kt (350 mph; 563 km/h) in lightly loaded condition and 289 kt (333 mph; 536 km/h) with full fuel and ammunition. This was a speed greater than that of the Hurricane Mk I but, as a result of its high wing loading, the prototype’s landing speed was comparatively high at 69.5 kt (80 mph; 129 km/h). The M.20/2 then suffered an accident at Woodley when landing on snow and was badly damaged.
In the meantime Miles had submitted the design of a fighter-bomber version, the M.20/3, capable of carrying a pair of 250-lb (113-kg) bombs under its wing, but this was not accepted. Instead, work had gone ahead as a private venture to produce a naval fighter version and this, the M.20/4, possibly converted from the M.20/2, first flew early in April 1941. The motivation behind this project was the threat posed by German long-range shipping raiders, and the resulting need for a low-cost eight-gun fighter capable of being catapulted from merchant ships to intercept the bombers. Fitted with a less cumbersome landing gear with smaller leg fairings and trimmed-down wheel spats, the M.20/4 would normally have had the main units removed once it had been mounted on its catapult for, in the event of being launched at sea, it would invariably be ditched at the end of its sortie.
The data for the M.20/4 differed from those of the M.20/2 in details such as its length of 30 ft 8 in (9.35 m), empty weight of 5,908 lb (2680 kg), maximum take-off weight of 8,000 lb (3629 kg), climb to 20,000 ft (6095 m) in 9 minutes 36 seconds, service ceiling of about 32,800 ft (9995 m), and range of 799 nm (920 miles; 1480.5 km).
The prototype was submitted to Specification N.1/41, which was in fact written round Miles’ proposal), and was delivered to the Aircraft & Armament Experimental Establishment at Boscombe Down on 18 April 1941, where it met with mixed reactions. Despite some improvements (including the lighter landing gear and a propeller spinner of improved shape) the maximum speed was still 289 kt (333 mph; 536 km/h). Catapult spools were incorporated under the centre fuselage. By this time, however, the Hurricane Mk I had proved suitable for catapult conversion and, being readily available in large numbers, had already entered service as the Sea Hurricane Mk IA. Development of the M.20/4 was then suspended and, after a brief visit to the Royal Aircraft Establishment Farnborough, the aeroplane was returned to Woodley on 19 March 1942 and scrapped in 1943.
Type: interceptor fighter
Accommodation: pilot in the enclosed cockpit
Fixed armament: eight (with provision for another four) 0.303-in (7.7-mm) Browning Mk II fixed forward-firing machine guns with 600 rounds per gun in the leading edges of the wing
Disposable armament: none
Equipment: standard communication and navigation equipment, plus a reflector gun sight
Powerplant: one Rolls-Royce Merlin XX liquid-cooled V-12 piston engine rated at 1,260 hp (939.5 kW) for take-off
Internal fuel: 154 Imp gal (184.9 US gal; 700 litres)
External fuel: none
Dimensions: span 34 ft 7 in (10.54 m); area 234.00 sq ft (21.74 m²); length 30 ft 1 in (9.17 m); height 12 ft 6 in (3.81 m)
Weights: empty 5,870 lb (2662 kg); normal take-off 7,758 lb (3519 kg); maximum take-off not available
Performance: maximum level speed 289 kt (333 mph; 536 km/h) at 20,600 ft (6280 m) declining to 250 kt (288 mph; 463 km/h) at sea level; cruising speed, normal 239 kt (275 mph; 442 km/h) at optimum altitude and economical 145 kt (167 mph; 268 km/h) at 9,000 ft (2745 m); initial climb rate 3,200 ft (975 m) per minute; service ceiling 31,400 ft (9570 m); maximum range 755.5 nm (870 miles; 1400 km); typical range 477.5 nm (550 miles; 885 km)