de Havilland DH-4B airplane
The DH-4B that hangs in the National Postal Museum, is actually a restored DH-4B, flown by the Airmail Service.

The DH-4’s design hails from de Havilland, a British aircraft manufacturer that went defunct in 1964. The DHs used by the airmail service, however, were American clones of the British design. As the United States had no adequate multi-role airframes entering World War One, the US produced massive numbers of the British DH-4, the Americans called it the "Liberty Plane."

The US Army had hundreds of DH-4s constructed, and when the end of the war came, the military had no further use for them. Fortunately, the Airmail service had a deal in place allowing them to help themselves to Army surplus airplanes.

Early results of experiences with the DHs could be charitably described as negative. Placement of the pilot’s seat and fuel tank would often lead to the pilot being crushed between the engine and the fuel tank in the event of a rough landing. The problem was exacerbated by the DHs’ weak fuselage and landing gear configurations. This lead pilots to call the DHs “Flaming Coffins” or “Damn Hearses.”

The Post Office Department, not wanting to scrap perfectly free planes, hired the Lowe, Willard, and Fowler Company to modify the DHs into something usable by the Airmail Service. Despite the fact that the L.W.F. failed to deliver on schedule, the modified DHs were successful beyond all expectations. Not only were the structural problems fixed, but the relatively weak engines that the DH-4s were fitted with were replaced by 400 horsepower Liberty engines. It was redesignated the DH-4B The DH-4B became the standard-issue mail plane, until they were replaced by the Douglas M-4 in 1926.

A twin-enine model of the DH-4 was also produced. The Twin DH was, by any standard, a horrendous mistake. L.W.F., at some point during their modifications of the Post Office’s DH-4’s decided to attempt a twin-engine variant. Praeger was impressed with the idea and ordered twenty of the Twin DHs.

The decision did not serve Praeger well. The modifications to DH-4 that made it into a Twin DH extended the wingspan, increased the cargo capacity and added new engines. The Twin DH had two engines that output 200 horsepower each, whereas the original had a single engine that output 400 horsepower by itself.

This increase in weight with no change in power caused the Twin DH to be severely underpowered. The Twin DH could barely maintain altitude at maximum speed, and taking off was said to always be an adventure. The Airmail Service quickly washed its hands of this mistake and sent the Twin DH where it belonged: the ash heap.