The Aircraft
The DC-3 was a development of the earlier DC-1 and -2, with each one getting a little larger and faster. The DC-1 was a result of a request by TWA for a Douglas airliner, as United had the production of the Boeing 247 sewed up and TWA needed a suitable airliner. The original aircraft after the DC-2 was the DST, Douglas Sleeper Transport, built for American Airlines to replace their Curtiss Condors. The DST first flew on December 17, 1935, the 32nd anniversary of the Wright Brothers first flight.
The huge advantage the DC-3 had was its greater speed and range. You could fly New York to LA in 17 hours, with 3 fuel stops. LA to New York was 15 hours due to prevailing winds. Previously, the practice was to fly passengers partway, put them on a train at sundown, and back on another plane at dawn.