The decade of the 1930s saw a proliferation of monocoque and semi-monocoque aircraft fuselage designs that broke aeronautics out of the post-World War I doldrums.
From the end of the war in 1918 until around 1928, successful aircraft designs were iterative advances on old concepts. Fuselages typically were made of underlying wood or welded steel tube frameworks on which fabric or lightweight plywood panels supplied more streamlining than structure.
Monocoque construction — French for single shell or single hull — was used to create strong, lightweight wooden boats before its adoption in the race-winning Deperdussin monoplane of 1911. In this type of construction, the skin is an important part of the load-bearing ability of the structure. Early attempts at adapting aluminum to monocoque aircraft structures in Germany struggled with metal consistency.
In the U.S., military procurement favored the traditional construction methods after World War I. Then, in 1928, the Air Service awarded Boeing a contract for an unusual monoplane fighter with a semi-monocoque fuselage, the XP-9.
Though performance did not warrant production, the experience with the XP-9 helped give Boeing the confidence to embrace monocoque construction for more designs, including the elegant Monomail that actually preceded the XP-9 in flight.
The Monomail targeted the then-popular market for single-engine mail and passenger planes. Its oval cross-section metal fuselage was mated to a low-mounted fully cantilever metal wing. The main landing gear retracted aft into wing recesses, leaving only part of the wheels exposed, substantially lessening air resistance. The clean single-wing design shed the drag of its traditional biplane ancestors, and the Monomail enjoyed a cruising speed of 135 miles per hour — between 25 and 30 miles faster than a contemporary Ford Trimotor.
It seems curious today, but in 1930, enough pilots voiced a preference for open cockpits that the otherwise streamlined Monomail featured an open seat for the pilot.
The Monomail design used stout square aluminum tubing to make trusses for the cantilever wing spars. Concepts in airframe design that were proven on the Monomail would resonate with subsequent Boeing designs that shaped the company’s destiny throughout the 1930s.
When the Monomail first flew on May 6, 1930, with Eddie Allen at the controls, its purposeful streamlining could not deliver the optimal performance of the design due to the use of ground-adjustable propellers of the day. Those propellers had to be set either for high-speed flight efficiency or takeoff power, and the takeoff performance was required for decent load carrying, especially from some high altitude airfields. It wasn’t until 1933 when Hamilton-Standard’s controllable pitch propeller would catch up with the streamlining airframe advances promised by semi-monocoque and cantilever metal construction techniques.
The Model 200 Monomail was a one-off prototype, as was the longer Model 221, also called Monomail. The original Model 200 was designed for mail and freight only; the Model 221 had six passenger seats in its lengthened fuselage.
The Model 200 came back to the factory for upgrade to Model 221A status, carrying eight passenger seats in a stretched fuselage.
Even though both Monomails were essentially prototype aircraft, they were placed in service on Boeing Air Tansport routes where they were given a thorough workout in operational mail and passenger tasks.
Although the two Monomails did not engender a long production run, Boeing designers were quietly relentless in their pursuit of clean metal airframes following the debut of the Model 200. The ensuing B-9 twin-engine bomber of 1931 made a design mockery of the contemporary Keystone biplanes then in Air Corps service.
But other manufacturers were wise to the benefits of all-metal monocoque construction, and the B-9 and its handful of derivations lost a production contract to Martin for the metal B-10 as competitors strived for ever better designs.
The march was on. Boeing doubled down on its twin-engine all-metal bomber design with the Model 247 airliner of 1933. The first American low wing multi-engine airliner, the 247 set the pace until Douglas overtook it with the more capacious DC-2 and DC-3 series later in the decade.
The cumulative result of Boeing’s design rationale reaching back to the Monomail was the Model 299 B-17 Flying Fortress — still low wing, still with square-tube truss structure in the wings, and originally with a tail design reminiscent of late B-9s and the 247 airliner.
Flown in company prototype form in 1935, the Model 299 enjoyed a long production run for aircraft of its era, closing out in 1945 with the delivery of the last B-17G Flying Fortress.
Boeing’s iterations of semi-monocoque aircraft designs from the Monomail to the B-17 shared a tailwheel stance and relatively broad chord wings. By the time the B-29 Superfortress was developed, the switch was on to high aspect ratio wings and tricycle gear as the aeronautical tools of the next design era were embraced.
