Source: Based on historic American Airlines and TWA timetables.
Introduced in 1936, the Douglas DC-3 was the first airliner able to undertake the profitable transportation of passengers. It was the outcome of specifications laid down by American Airlines for an airliner that could offer coast-to-coast “sleeper” overnight services. When configured for such operations, the aircraft had a capacity of 14 passengers; but in a normal seating configuration, it held 21.
As the above map indicates, the DC-3, with an operating speed of 350 km/hr could cross the continent in about 15 hours, with three refueling stops along the way. By the late 1930s, American’s DC-3 “Mercury” service was competing with the slightly faster DC-3 Sky Chief operated by TWA (Transcontintental & Western Air, whose name was change to Trans World Airlines in 1950). The eastbound service was organized as an overnight transcontinental crossing so that passengers could arrive on the morning of the next day. The westbound Mercury flight would leave New York at 5:10 PM and arrive in Los Angeles at 7:51 AM.