With the Spitfire and Hurricane in widespread service, the Air Ministry contemplated their eventual replacement. Specification F18/39 issued on May 1, 1939, called for a single-seat interceptor armed with two cannon or eight machine-guns, capable of 400mph (644km/h) at 12,000ft (3,658m) at an all-up weight of 15,000 lb (6,800kg), and with a ceiling of 35,000ft (10,668m).
In the summer Martin-Baker received an order for three prototypes of Martin's new MB.3 at a total contract price of £64,000. With war imminent Martin had been asked to 'come aboard' and produce prototypes to contract as had established contractors in the earlier F5/34 submission.
In its fuselage structure the MB.3 was an advance on its predecessors. The skinning was of stressed light alloy and flush-riveted except where the numerous inspection panels were situated. The wing was completely new, its basis being a D-section torsion box. A substantial light alloy spar web was adopted, with upper and lower spar booms made from laminated high-tensile steel strip, the number of laminations decreasing towards the tip. The structure was covered with heavy gauge flush-riveted light alloy stressed skin. The fin was reminiscent of that fitted late in the life of the MB.2. All control surfaces were fitted with torsion-bar spring tabs to enhance manoeuvrability.
Martin had intended a Rolls-Royce Griffon as the powerplant, this being the engine specified in F18/39. As an interim measure, the company was supplied with a Merlin ballasted to simulate the Griffon. With the advent of the Napier Sabre, which promised to give over 2,000hp (1,492kW), a second option became available and in 1941 Martin-Baker was awarded a revised contract for a Napier Sabre II version of the MB.3. The Sabre drove a de Havilland three-bladed constant speed propeller. Shallow ducted radiators were incorporated under the left wing (oil cooler) with the main radiator under the right wing.
The main inward retracting undercarriage of the MB.3 had an track of 15ft 5ins (4.7m). The wheels were raked so far ahead that the threat of a nose-over was minimal. Undercarriage operation was by 60 lb/sq.in (27kg/2.5csq.m) pneumatic system. A second, lower pressure pneumatic system was used to operate the large split flaps which had no intermediate settings.
The intended armament consisted of six 20mm Hispano cannon, to have 200 rounds per gun. The armament was removed prior to flight trials and never tested.
The prototype MB.3, R2492, emerged from Denham in the summer of 1942, the contract completion date having been set for the previous March. Initial taxiing trials were undertaken at Denham and demonstrated a very stable ride.
Martin received orders from the Ministry of Aircraft Production (MAP) to use RAF Wing in Buckinghamshire for flight testing and during August 1942 the MB.3 arrived at Wing in crates, Val Baker flying it for the first time on 31 August 1942. According to an eye-witness, the MB.3 with its 24-cylinder sleeve-valved Napier Sabre flew well for a short period but then had to make a hurried landing because of the threat of engine overheating due to inadequate radiator cooling.
In subsequent flights in early September, Baker found the MB.3 highly manoeuvrable. On September 12, 1942, Valentine Baker took off satisfactorily, but shortly afterwards the Sabre failed when a sleeve drive-crank broke, resulting in immediate and total loss of power. Baker attempted a dead-stick landing but after touching down in a small field, the MB.3 hit a tree stump and was destroyed; Baker was fatally injured.
After the loss of R2492, the MB.3 prototype programme came to a halt. Martin decided to start again with a new design. It had been by no means certain that the MB.3 would have secured a production contract.
Engine: One 2,020hp (1,506kW) Napier Sabre II 24 Cylinder H-type piston
Max speed 430 mph (691km/h)
Range 840 miles (1,351 km)
Initial climb, 4,350 ft/mm (22,09 m/sec)
Operational ceiling 34,500ft (10,515m)
Empty weight 8,500 lb (3,855kg)
All-up weight 11,000 lb (4,989kg)
Wnigspan 35ft 2in (10.7m)
Wing area, 262.64 sq ft (24,40 sq.m)
Length 34ft 1 in (103m)
Height: 14ft 3in (4.3m)
Armament: Six Hispano cannon intended.