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.