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Boeing Model 40 - United States, 1927

Detailing the development and operational history of the Boeing Model 40 Mailplane Transport Biplane.

 Entry last updated on 4/25/2016; Authored by Staff Writer; Content ¬©

  Boeing Model 40  
Picture of Boeing Model 40
Picture of Boeing Model 40 Picture of Boeing Model 40Picture of Boeing Model 40Picture of Boeing Model 40Picture of Boeing Model 40

Fewer than 100 Boeing Model 40s were produced in the latter part of the 1920s - these serving with a select few global operators.

One of the earlier, effective uses of the airplane outside of the military realm was in mail delivery. Until this point in history, mail arrived view rail or other ground-based method. Some aircraft designs were developed exclusively for the mail delivery service and generally built around good speed and inherently strong hauling capabilities. Boeing did just that with its Model 40 which undertook its first flight on July 20th, 1925 and introduced the product in July of 1927. About 80 of the type were ultimately built and served several transport lines of the day (including Boeing Air Transport).

The aircraft was originally developed to a new requirement put forth by the United States Postal Service (USPS). Up to this point, the service relied on the British de Havilland DH.4 biplane which appeared during the fighting of World War 1 (1914-1918) in 1917. Nearly 6,300 of the two-seat biplane light bombers were produced with a bulk of this (including its Liberty engines) emerging from American factories. The line went on to influence subsequent aircraft such as the DH-9 detailed elsewhere on this site.

To ease development and production, the same Liberty engines used by the wartime version of the aircraft would be featured in the new mail plane. A biplane wing arrangement was retained and the tail unit sported a conventional single-finned configuration. The open-air, single-seat cockpit was seated aft and under the upper wing element. The undercarriage was fixed and wheeled at all three legs. The exposed Liberty V12 radial engine was fitted in the nose and drove a two-bladed propeller. Steel tubing, aluminum, wood and fabric were used in the aircraft's general construction makeup. The original aircraft held a cargo capacity of 1,000 lb.

The resultant design was the Boeing "Model 40". The USPS ended up pursuing the competing Douglas M series instead after securing the Boeing prototype. However, the Contract Air Mail Act of 1925 opened two fronts in the mail service approach, an eastern and western region, and Boeing looked to secure its aircraft for the latter routes. This involved revising the Model 40 into the "Model 40A" to make for a more tempting product in which a key change was introduction of the lighter weight Pratt & Whitney "Wasp" radial engine of 425 horsepower output. An interesting addition was a two-person cabin fitted between the upper and lower wing elements where each position was given a hinged, automobile-style access door and viewing windows. In terms of cargo, the aircraft improved its hauling capacity to 1,200 pounds of mail goods.

Twenty-five Model 40A aircraft were purchased and some were eventually fitted with Pratt & Whitney "Hornet" engines of 525 horsepower for improved performance. This produced the Model 40B-2 variant which were essentially re-engined Model 40A aircraft. The Model 40B-4 designation marked new production aircraft fitted with Hornet engines from the outset. The Model 40B series was followed by the "Model 40C" which could seat four persons in its cabin space.

Beyond the United States, the Model 40 was featured as part of the Honduran Air Force service and a pair are known to have been delivered to New Zealand. Boeing Canada added aircraft to the total production stock through a few of its Model 40H-4 variant.

Performance from this interwar biplane (in particular the Model 40A) included a maximum speed of 128 miles per hour, a cruise speed of 105 miles per hour, a range out to 650 miles, a service ceiling of 14,500 feet and a rate-of-climb of 770 feet per minute.

Boeing Model 40 Specifications

Service Year: 1927
Type: Mailplane Transport Biplane
National Origin: United States
Manufacturer(s): Boeing - USA
Total Production: 80

Structural (Crew, Dimensions, Weights)

Operating Crew (Typical): 1
Overall Length: 33.14 feet (10.1 meters)
Overall Width: 44.29 feet (13.50 meters)
Overall Height: 12.30 feet (3.75 meters)

Weight (Empty): 3,538 lb (1,605 kg)
Weight (MTOW): 6,019 lb (2,730 kg)

Power / Performance (Engine Type, Top Speed)

Engine: 1 x Pratt & Whitney Wasp radial piston engine developing 420 horsepower.

Maximum Speed: 111 knots (127 mph; 205 kph)
Maximum Range: 564 nautical miles (649 miles; 1,045 km)
Service Ceiling: 14,501 feet (4,420 meters; 2.75 miles)
Rate-of-Climb: 770 feet-per-minute (235 m/min)

Armament / Mission Payload


Global Operators (Customers, Users)

Canada; Honduras; New Zealand; United States

Model Variants

Model 40 - Base Series Designation; original model of 1925 with Liberty powerplant.
Model 40A - Fitted with PW Wasp radial engine; two-seat passenger compartment; 25 examples.
Model 40B - Fitted with PW Hornet radial engine
Model 40B-2 - Re-engined Model 40A with Hornet engine; 19 examples.
Model 40B-4 - Four-passenger cabin seating; 38 examples completed.
Model 40B-4A - Engine testbed airframe
Model 40H-4 - Boeing Canada Model 40B-4; four examples
Model 40C - PW Wasp engine; ten examples
Model 40X - Special variant based on Model 40C with two passenger seating in cabin; forward cockpit ahead of pilot.
Model 40Y - Special variant based on the Modle 40X; PW Hornet engine used.

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