At the outset Martin considered the best decision was to build a machine virtually identical with the M.B.3, but with a Rolls Royce Griffon engine. By the end of 1942 the Griffon engined M.B.4 had been almost finished as far as drawings were con¬cerned, but it contained a number of unsatisfying compromises. Early in 1943 Martin took the decision to start again and, while making the maximum possible use of MB.3 philo¬sophy and hardware, create an optimised fighter that would have no obvious shortcomings.

The wing of the M.B.5 was similar to that of the M.B.3, though Martin reduced the armament to four 20mm Hispanos, with his own patented flat ¬feed mechanism, which met the F.18/39 specification. The wing was simpler than that of the M.B.3, with light alloy sheet structure except for the spar booms which were of laminated high ¬tensile steel. The fuselage philosophy was the same but the shape was considerably different, and the steel ¬tube longerons were joined by flanged sleeves to which the diagonal, vertical and transverse struts were secured by fork ends held by close tolerance tapered bolts. Light alloy secondary structure supported heavy section rubber mouldings against which the large skin panels were pressed by Dzus fasteners. Control surfaces were metal skinned and driven by spring tabs, and the landing gear, brakes and up/down flaps were worked by compressed air (not at 60 lb/sq in but 350, giving a lighter system).

The construction of the instrument panel itself was arranged for ease of maintenance. Mounted on a sub frame, it was designed to hinge open for access to the interior and the instruments themselves. The position of the cockpit afforded an excellent view over the nose and wing leading edges. The cockpit also included a full floor, which in 1944 was rather novel.

The pilot sat on fuel tanks of 70 and 130 Imp gal, and the Griffon 83 engine had its air intake just behind the spinner of the Botol six blade contra rotating propeller, and was cooled by a three unit radiator inter-cooler, main cooler and oil cooler ¬in a neat diffuser duct under the rear fuselage. The propeller shafts were mounted coaxially, the drive for the forward one passing through that for the rear. Contra rotation had been chosen to provide greater performance at altitude, and to permit higher power at take off than would otherwise have been feasible.

A Rolls Royce Bendix Stromberg injection carburettor was adopted, the long carburettor air intake extending up to the spinner and forming an integral part of the engine cowling; vertical ‘splitters’ ensured stiffness and reduced turbulence.

The three cooling radiators needed were placed in a diffuser duct under the rear fuselage. In the forward position was the intercooler, aft of this the main radiator and finally the oil cooler. This arrangement ensured that, when warming up, the heat from the main radiator prevented coring of the oil, and the full oil pressure was obtained within a few minutes of start¬up. Fuel was contained in two tanks of 70 and 130 imp. gallons (318 and 590 litres) situated one ahead of and one behind the pilot.

The MB.5 prototype received the serial R2496, originally allotted for the second MB.3 (which was never completed). On the morning of May 23, 1944, R2496 was ‘knocked down’ at Denharn in about an hour, placed aboard a Queen Mary road transporter and taken to Harwell.

Reassembled in just over an hour, the MB.5 was inspected and flown for the first time in the afternoon by Captain Bryan Greensted, Rotol’s chief test pilot, who had been seconded to the programme by MAP.
During the company testing Greensted found the MB.5 well thought out and was impressed by all but the directional handling. Fin and rudder modifications were swiftly made involving an extension to the fin leading edge and a larger and taller rudder. With these modifications, W/C Maurice Smith, who later briefly flew R2496 for Flight magazine found that the handling was “pleasing in every way”.

Towards the end of October 1944, after 40 hours in the air, R2496 was taken to Farnborough for demonstration by Bryan Greensted before visiting VIPs, including Winston Churchill. During this display the Griffon failed when a piston shattered. As his cockpit filled with smoke, Greensted reached up to jettison the canopy, while travelling at a speed of over 400mph (644km/h). As he did so, his arm became caught in the slipstream and was very severely dislocated. Meanwhile the canopy had clipped the tailplane causing slight damage. Greensted elected to remain aboard, however, and managed a deadstick landing. As a result of this experience, the offending hood jettison lever was repositioned.

In early 1946, with 80 hours 45 minutes flown, the MB.5 was handed over to A&AEE at Boscombe Down. The first testing report, on maintenance and accessibility was issued on March 1. Prominently at the beginning it stated, “It is considered that the general design and layout of the Martin Baker 5 is excellent, and is definitely better from the engineering and maintenance aspect than any other similar type of aircraft”. It went on “The layout of the cockpit might very well be made a standard for normal piston engined fighters” and “The time necessary for a quick turn around… would appear to be very low when compared with existing types of aircraft”.

Flight test comments passed by Boscombe praised R2496 as virtually viceless. It was found that the contra prop simplified take off and gave an immediate and powerful response to engine power changes, over a very wide speed range. Minimal engine vibration was experienced, and the cockpit noise level was low. Both the rudder and elevators were light, and control was exceptional especially near the stall, which was very gentle. Use of the trimmer was found necessary only infrequently; even at low speed with flaps down, when the aircraft became mildly tail heavy, it was not sufficient to warant moving the trimmer from its usual neutral position. The only mildly adverse comment was over a slight lateral instability caused by comparatively ineffective ailerons the rate of roll was mediocre but could be improved by use of rudder. Lt Cdr Eric M Brown, doyen of Service test pilots described R2496 as “superb”.

Flaps of the split trailing edge type are either selected up or down, no inter¬mediate position being provided, so a take off is made without flap. The mini¬mum take off for the M B V is 420yd over 50ft in zero wind. Wheels retract quickly, the only indi¬cation that they have moved and locked being that given by the warning lights. The throttle lever, placed above the pitch control, has a long quadrant and moves smoothly and positively. The pitch lever is, by comparison, rather sensitive, and a big change in rpm results from quite a small lever movement.
Engine speed is controlled over a range of roughly 1,800 rpm to 3,000 rpm, and the blades cannot be feathered. At about 8,000 ft the M B V cruised along very sweetly at about 315 mph indicated with 4 lb boost and 2,250 rpm. This is somewhat less than the recommended cruising power and the speed is at least 100 m.p.h. less than the maximum for this height.

All of the controls have spring tabs.

With flaps down, engine idling and attitude very slightly nose up, the con¬trols became pretty sloppy at 110 mph indicated at 5,000ft and the aircraft stalled very gently away to the left at just over the 100 mph.

The sole prototype MB.5 passed from hand to hand attracting praise wherever it went. In June 1946, it was demonstrated at Farnborough by S/L Jan Zurakowski. After this, the MB.5 returned to Boscombe, where the Commandant, Air Commodore Sir Henry Paterson Fraser, flew it. He found R2496 free of vice. The MB.5 was popular and unique. Later, R2496 was taken to the newly acquired Martin Baker airfield at Chalgrove. There, it was used to examine oiling problems in the propeller translation bearing between the pair of blade sets.

Martin took out a patent for the automatic oiling unit developed as a result of these trials. It continued in use in the de Havilland contra rotating propellers of the Avro Shackleton. The MB.5 was maintained in an airworthy condition, flight testing continuing into 1947, the sole prototype being scrapped thereafter.

Engine: One Rolls-Royce Griffon Mk 83 piston engine with two stage, two speed supercharger
Maximum power, 2,340 h.p. at 750ft (M gear), 2,120 h.p. at 12,250ft (S gear) with 25 lb/sq in boost
Max speed: 460mph (740kph) at 20,000 ft (740 km/h at 6,096m)
Stalling speed (assumed ISA, SL) 95 mph (153 km/h)
Range at 225 mph (362 km/h) 1,240 miles (2,000 km)
Service ceiling: 40,000ft (12,192m)
Empty weight 9,233 lb (4,188kg)
Empty wt, equipped 9,345 lb (4,230kg)
All up wt: 11,500 lb (5,216kg)
Wingspan: 35ft (10.6m)
Length: 37ft 9in (11.5m) with final tail unit
Height: 15ft (4.5m), with final tail unit
Wing area, 262.64 sq ft (24,40 sq.m)
Wing loading: 44 lb/sq ft
Armament: Four 20mm Hispano cannon with 200 rpg.

The second of the three MB.3 prototypes ordered by MAP, R2496, was intended to be Griffon powered, and was referred to as the MB.4. At the time of R2492’s crash, the necessary drawing amendments for the MB.4 were complete and manufacture had started.
It was intended to enter the competition against Specification F6/42 but the attempt was unsuccessful. However, Martin felt that he could improve on the MB.3/MB.4 concepts. R2496 was left uncompleted and work on the third intended aircraft, R2500, was not begun. Early in 1943, the difficult decision was taken to start afresh, tempered by the knowledge that the firm had received verbal assurances from MAP that if a new fighter design were produced, a contract for one prototype would be awarded, again to Specification F5/39.
With the then hand built nature of a prototype, in an experimental shop, with design adjustments made on the spot, and drawings brought into line afterwards, made resources available out of all proportion to the effort. By 1943, when work on the MB.5 had begun.

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.