The triple-tail mystery plane is the Douglas DC-4E prototype which first flew on June 7, 1938. I remember it well since my birth date was a mere 8 days later. Much of the test flying was done by Benny Howard. This ship began an extensive nationwide shakedown tour by United Air Lines in November of 1939 which proved that changes were needed. The production Douglas DC-4/C-54, which was slightly shrunk and sported a single vertical stabilizer, was a wonderful airplane as we know, but the prototype was unloved. It was sold to Japan Air Transport in late 1939 and later crashed. (Info courtesy of Jane's All The World's Aircraft vols.1939 and 1940, and Juptner's U.S. Civil Aircraft, vol. 8.)

Douglas DC-4E - predecessor to the actual DC-4 and C-54 transports of World War II fame. The '4E was just too complex, too expensive ($1.6 million each), and too far ahead of its time when it rolled out in 1938, and although it was beautifully flush-riveted and was the first airliner "with provisions for pressurization" (the Boeing 307 was first to fly with pressurization), it was basically a scaled-up four-engined DC-3. Although those upper fuselage windows gave the appearance of an upper deck, those were actually for the sleeper berths that folded down from the craft's ceiling. The lone prototype was sold to the Japanese in 1939, and served as the baseline for their bomber fleet that appeared shortly thereafter.