Using lessons learned from the MB1 and MB2, James Martin and Captain Valentine Baker capitalised on the design and construction to develop a new design, the MB3, which was powered by a 2,000 hp Napier Sabre 24-cylinder, H-type engine, driving a de Havilland variable-pitch three-blade propeller.
The MB3 was projected to meet an Air Ministry fighter specification. It was armed with six 20 millimetres (0.79 in) cannon mounted in the wings, each with 200 rounds of ammunition, which made it the most heavily armed fighter in existence and for ease of maintenance the armament was easily accessible. Three were ordered to specification F.18/39 which was written for the design.
Length: 10.77 m
Wingspan: 10.7 m
Height: 4.5 m
Wing area: 24.3 m²
Empty weight: 4,188 kg
Loaded weight: 5,216 kg
Max. takeoff weight: 5,484 kg
Powerplant: 1 × Napier Sabre 24-cyl, H-type engine l-c engine, 2,000 hp (1,745 kW)
Maximum speed: 415 mph (668 km/h) at 20,000 ft (6,100 m)
Range: 1,770 km
Service ceiling: 12,190 m
Rate of climb: 19.3 m/s
Guns: 6 × 20 mm Hispano Mk.II cannon
While retaining the essential characteristics of the earlier designs, MB3 included many new features: the fuselage primary structure was still the round steel tube arrangement but metal panels had taken the place of wood and fabric of earlier models. The wing construction integrated torsion-box construction and a laminated steel spar, giving a strong and stiff structure with minimum flexing. Attention to detail extended to a Martin-designed pneumatically-controlled undercarriage that was simple, sturdy, effective and reliable. With the wing flaps also pneumatically operated, the need for hydraulics, with all their attendant operational hazards and maintenance problems, was eliminated. Underwing radiators had the coolant radiator on the starboard and the oil cooler on the port side.
Then listed as "Experimental Aeroplane No.120" and with the serial number R2492, the MB3 was temporarily stationed at 26 OTU (Operational Training Unit - RAF Wing in Buckinghamshire) for trials and first flew on 31 August 1942. The tests were supervised by Group Captain Snaith and observed by, amongst others, Air Vice Marshals Linnell and Burton. Following its successful first flight, undertaken by Captain Baker, the next series of test flights revealed the MB3 to be highly manoeuvrable and easy to fly but on 12 September 1942, the engine failed soon after take-off and Captain Baker, in an attempt to save the aircraft while executing a difficult forced landing, crashed in a field and was killed.
Capt. Baker had just got airborne, when he had immediate loss of power. In trying to save the aircraft he made a forced landing in a field but hit a tree stump and was killed.
The subsequent Court of Inquiry found the cause to be "...engine failure due to a broken sleeve drive crank in the engine." Rumours suggested that the Napier Sabre engine used had been overheating on the ground. Ground-crew George Bignall recalled "Although nobody was allowed into the aircraft hangar, we were able to see it during its testing time. It was very fast with Captain Baker flying very close at times, shooting up the runway very low.
" Of the day of the accident he said "I was doing a modification on dispersals when Captain Baker took off towards Stewkley. I watched him climb then suddenly the engine cut out and he crashed, trying to land."
Civilian John Thornton also witnessed the accident,
Two fields from where Morris and I were harvesting, there was a stack of newly-threshed straw. The MB3 hit this and burst into flame. 'Bunny' Winter, the bailiff of Cold Harbour Farm, beat us to the crash, but we were too late to rescue Captain Baker in the fierce fire.
The Unit Report states that Mr Winter actually managed to remove Capt. Baker's body from the aircraft.
Due to various delays and late delivery, the Ministry considered the design outdated and there was no production order.
The MB3 design could have been developed into a good fighter aircraft. Martin felt the personal loss of his best friend and partner and "many consider that it was this painful tragedy that really fired the passionate interest in the safety of aircrews, which was later to become the very pivot of his life." He devoted the remainder of his life to the invention and development of the successful Martin-Baker ejection seats (the company retained the name Martin-Baker as a tribute to their lost co-founder).