Boeing Type 40B-4 “Mailplane”
United States — single-engined mail and passenger-carrying biplane

Archive Photos [1]

1927 Boeing 40B-4 (NC285) at the Henry Ford Museum, Dearborn, MI

Overview [2]

  • Boeing Model 40B-4
  • Role: Mail-passenger
  • Manufacturer: Boeing
  • First flight: 5 October 19328

The Boeing Model 40 was a United States mail plane that became the first aircraft built by the Boeing company to carry passengers. It was of conventional biplane configuration with a combination of standard and warren-truss style interplane struts. Originally designed to compete for a US Mail contract in 1925, it was rejected in favor of the Douglas M-2.

The design was revived in 1927 as part of Boeing's tender for newly privatized airmail routes. Designated the Model 40A, this variant was powered by an air-cooled Pratt & Whitney Wasp radial engine, which offered a 200 lb weight saving over the water-cooled Liberty specified by the postal service in 1925. Although the primary purpose of the aircraft was to carry mail, two passengers could be accommodated in the small cabin, allowing Boeing to operate it on any of the routes that the firm might bid for. The original fuselage design was changed to one using welded steel tubing. Boeing successfully bid on the San Francisco-Chicago route, and Boeing Air Transport commenced operations on 1 July 1927 with 24 Model 40As.

Survivors [2]

  • As of February 17, 2008, Boeing 40C S/N 1043 became the only airworthy example in the world. It also holds the title of the oldest flying Boeing in the world. In 1928, the aircraft was substantially damaged in a crash and was totally rebuilt over a seven year period and an estimated 18,000 man hours by Pemberton and Sons Aviation in Spokane, Washington.
  • The Henry Ford Museum in Dearborn, Michigan, contains a 1927 Boeing 40B-2, number 285.
  • The Museum of Science and Industry in Chicago, Illinois has a 1928 Boeing Model 40-B on display in its Transportation Gallery. (N288)
  • The Museum of Flight in Seattle, Washington has a complete full-scale replica and two partially finished replica fuselages (showing what the original Boeing factory would have looked like circa 1928-29) on display.

Variants [2]

  • Model 40: Original 1925 design with Liberty engine.
  • Model 40A: Revised 1927 design for BATC. the aircraft was powered by a Pratt & Whitney Wasp radial engine, plus seating for two passengers in an enclosed cabin; 25 built.
  • Model 40B: Model 40As re-engined with a 525-hp (391-kW) Pratt & Whitney Hornet radial piston engine. 19 Model 40A were converted. Redesignated Model 40B-2.
  • Model 40B-4: Revised Model 40B with seating for four passengers and other improvements. Equipped with openable windows, plus seating for four passengers; 38 built.
  • Model 40B-4A: One Model 40B used as engine testbed by Pratt & Whitney.
  • Model 40H-4: Four Model 40B-4s built by Boeing Canada. Two aircraft were exported on New Zealand.
  • Model 40C: Similar to Model 40B-4 but with Pratt & Whitney Wasp engine of Model 40A. (10 built, all later converted to Model 40B-4 standard).
  • Model 40X: Unique special-order machine similar to Model 40C with only two-passenger cabin and extra open cockpit forward of pilot's cockpit.
  • Model 40Y: Unique special-order machine similar to Model 40X, but with Pratt & Whitney Hornet engine.

Operators [2]

  • United States: Boeing Air Transport

Boeing Model 40B-4 Specifications [3]


  • Single-engined mail and passenger-carrying biplane.


  • Equal span biplane, top plane flat, bottom planes with dihedral.
  • Top wing carried on inverted Vee cabane, over fuselage.
  • From the points of attachment of cabane struts on top longerons, short struts slope down to bottom wings, and from their points of attachment to the bottom plane, two sets of slightly splayed interplane struts run up to the top wings, one on either side of the fuselage.
  • At each wing-tip there is one set of interplane struts.
  • Wing construction consists of solid spruce spars, routed out for lightness, spruce ribs and a plywood leading-edge, the whole being covered with fabric.
  • Skew-type ailerons fitted to all for planes.


  • Welded steel-tube structure, covered with fabric.

Tail Unit:

  • Normal type.
  • Welded steel-tube framed, covered with fabric.


  • Split type.
  • Consists of two steel-tube Vees, at the apices of which are sprung the two crossed bent axles, the upper ends of which are hinged to the bottom fuselage longerons.

Power Plant:

  • One 525 hp Pratt & Whitney “Hornet” air-cooled radial engine.
  • Fuel is carried in two tanks, one of 60 U.S. gallons (50 Imp. galls. = 227 L) capacity, in the fuselage, between the front mail compartment and the cabin, and the other, of 40 U.S. gallons (33 Imp. galls. = 151 L) capacity, in the top plane.
  • Petrol fed by gravity from the wing tank, with a hand pump provided if the normal pressure pump fails to raise fuel from fuselage to wing.
  • Engine-driven generator and storage battery provide current for lighting and for operating electrical inertia starter.
  • Pressure fire-extinguisher, covering both engine and petrol tank compartments, controlled by pilot, and hand extinguishers are provided in both cabin and pilot's cockpit.


  • Behind the engine bulkhead is a metal-lined mail compartment of 25 ft³ capacity, designed to carry approximately 500 lb of mail.
  • Behind this compartment is the passenger cabin, which is also metal-lined.
  • The seats of four are arranged in pairs, each behind slightly staggered, to provide ample shoulder-room.
  • Two doors, provided on left side, and all windows are glazed with non-splinterable glass and are openable.
  • Forced heating, ventilation and lighting are installed.
  • Fore an aft of the cabin there is space for the stowage of hand luggage.


  • Span: 44 ft 2¼ in (13.46 m)
  • Length: 33 ft 47/16 in (10.16 m)
  • Height: 11 ft 8½ in (3.56 m)
  • Wing area: 545 ft² (60.63 m²)

Weights and Loadings:

  • Weight empty: 3,809 lb (1,729 kg)
  • Payload: 1,171 lb (531.6 kg)
  • Weight loaded: 6,080 lb (2,760 kg)
  • Wing loading: 11.5 lb/ft² (54.4 kg/m²)
  • Power loading: 11.57 lb/hp (5.25 kg/hp)


  • Maximum speed: 135 mph (216 km/h)
  • Landing speed: 57 mph (91.2 km/h)
  • Initial rate of climb: 800 ft/min (244 m/min)
  • Climb in 10 minutes: 6,320 ft (1,927.6 m)
  • Service ceiling: 15,100 ft (4,600 m)


  1. Shupek, John. Photos via The Skytamer Archive, copyright © 2009 Skytamer Images. All Rights Reserved
  2. Wikipedia. Boeing Model 40, 4 December 2009
  3. Grey, C. G. and Leonard Bridgman. “The Boeing Type 40-B-4 Mailplane”. “Jane's all the World's Aircraft 1931”. London: Sampson Low, Marston & company, Ltd., 1931, p. 253c-254c. Print.

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