that transmitted noise and vibration back to the passengers, Douglas planes
With the
introduction of the Douglas  
DC-2 in 1934 and the DC-3 in
1936, air travel became much more comfortable and somewhat more commonplace.
The DC-2 could fly coast-to-coast faster than any passenger plane before, and
the DC-3 had both day and sleeper models, allowing passengers to travel
cross-country in comfort. By 1939, at least 75 percent of all air travelers
were flying on DC-3s. While the earlier trimotors had been plagued by engines
that transmitted noise and vibration back to the passengers, Douglas planes
added soundproofing to its cabins, ventilation ducts, and structure.
Upholstered seats mounted on rubber and padded arm rests further reduced noise
and vibration. The planes could also fly higher, around 20,000 feet, (6,100
meters), reducing, although not eliminating, turbulence, and the spar structure
made the cabin roomier and easier to navigate than the contemporary Boeing 247,
which had an internal spar that passengers had to step over.