The MB-4 was designed to meet a June 1919 specification from the United States Post Office Department for a two- or three-engined mailplane, required to carry 1,500 lb (682 kg) of mail on a single engine.^[1] Thomas-Morse chose to use as much as possible of its existing Thomas-Morse MB-3 fighter in order to reduce costs, using two engineless MB-3 fuselages to carry the crew and cargo, with the pilot in a cockpit in the nose of the port fuselage and the co-pilot/mechanic in a similar cockpit in the nose of the starboard fuselage, while cargo was carried behind the crew in each fuselage. Two 300 hp (224 kW) Wright-Hisso H V8 engines were mounted in a central nacelle between the two main fuselages in a push-pull configuration, with fuel tanks mounted between the engines.^[1][2][3]

The MB-4's three-bay biplane wings were all new, with ailerons on the upper wing.^[3] It had a conventional tailwheel undercarriage and had two separate tail assemblies, which were standard MB-3 empennages. Dual controls were fitted, with the pilot able to disconnect the co-pilot's controls, but there were no means of communication between the two cockpits.^[1][2]

The MB-4 made its maiden flight in February 1920.^[1] While the aircraft had reasonable speed for the time,^[3] it was otherwise poor, with one fuselage tending to take-off before the other, while the engines caused severe vibration which overloaded the aircraft's structure.^[1][4] It was described as the "worst thing on wings" by Jerome Fried, the general plant superintendent of Thomas-Morse.^[1]

One aircraft was tested by the US Post Office, but was not used for mail services, and was scrapped in 1921.^[4] At least one MB.4^[1] (and possibly 3^[4]) was delivered to the United States Army Air Service where it was stored before being scrapped.^[1]

The MB-4 was a failure, having extremely poor flying characteristics and being described as the "worst thing on wings",^[1] and saw no service other than the trials by the manufacturer, US Army and the US Post Office.

Data from General Dynamics Aircraft and their Predecessors^[1]

General characteristics

• Crew: 2
• Length: 25 ft 5 in (7.74 m)
• Wingspan: 45 ft 6 in (13.87 m)
• Height: 11 ft (3.4 m)
• Wing area: 645 sq ft (59.9 m²)
• Empty weight: 3,554 lb (1,612 kg)
• Gross weight: 5,564 lb (2,524 kg)
• Powerplant: 2 × Wright-Hisso H water-cooled V8 engines, 300 hp (220 kW) each

Performance

• Maximum speed: 122 mph (196 km/h, 106 kn)
• Stall speed: 53 mph (85 km/h, 46 kn) ^[2]
• Range: 600 mi (970 km, 520 nmi)
• Time to altitude: 9,000 ft (2,745 m) in 10 min

Aircraft of comparable role, configuration, and era

• Caproni Ca.5
• Grahame-White Ganymede
• L-W-F Model H Owl

Related lists

• List of Trimotors
• List of Twin-boom aircraft

Notes

• "Airisms from the Four Winds". Flight, 25 March 1920. p. 350. (Contains two photos of the MB-4).