This Week in Aviation History — de Havilland DH-4

In early July 1925, new purpose-built airplanes were required to replace the modified military de Havilland DH-4 aircraft, which had provided a start for flying mail. To fill the needs of the Post Office Department, two craft emerged — one from Douglas Aircraft, in California, and one from The Boeing Company, in Washington.

From Douglas came a design inspired by the sturdy Douglas O-2 observation plane used by the Army. Called the Douglas Air Mail One or DAM-1, it had the Post-Office-required Liberty V-12 engine, and in front of the pilot a metal and asbestos mail compartment for the safe carriage of 1,000 pounds of mail. The aircraft made its first flight July 6, 1925.

Sire of a line of airmail airplanes known as the M-2, M-3 and M-4, the DAM-1 had powerful landing lights and provisions for parachute flares. The original biplane had no seating outside of that required for the pilot, but later variants had seats for two, with the 400hp Liberty allowing for a greater speed and range than the Havilland DH-4s it replaced. The Douglas carried two and a half times the payload of the DH-4 — easily.

The second aircraft designed for the airmail task was the new Boeing Model 40. Larger than the competing DAM-1, the 40 carried the same 1,000 pound payload at a slower speed and lower ceiling, but a longer range. First flown on July 7, 1925, the Boeing 40 was not selected by the Post Office, the government retaining a single example.

But by August of 1927 the Post Office was no longer flying the mail, the duties having been passed to the ancestors of today’s airlines. The Douglas M series were operated by National Air Transport and Western Air Express, while Boeing Air Transport operated the revised Model 40, the 40A. With the Liberty engine replace by a Pratt & Whitney Wasp radial, the 40A now carried a bit more; 1,200 pounds of mail, two passengers in a small cabin, plus the pilot.

Earlier in 1927, the first certification regulations were put in place, and the 40A brought Boeing Approved Type Certificate No. 2. The 40A and subsequent variants ultimately served with several carriers and firms, and export versions went as far afield as Australia for transport work. Which isn’t bad for a design that began as a last place finisher in an airmail contest 75 years ago that helped pave the way for today’s global air system.