Douglas DC-3 Airliner.
No greater accolade for the DC-3 exists than the fact that over six decades after its first flight more than 400 remain in commercial service worldwide. Durability, longevity and profitability are but three of this outstanding aircraft's virtues.
Development of the DC-3 traces back to the earlier oneoff Douglas Commercial 1 (DC1) and subsequent DC2 which made their first flights in 1933 and 1934 respectively. In 1934 American Airlines requested that Douglas develop a larger more capable version of the DC2 for transcontinental US sleeper flights. The resulting DC-3 (or DST - Douglas Sleeper Transport as it then was) flew for the first time on December 17 1935.
An almost instant sales success, the DC-3 became the mainstay of the US domestic airline network in the years prior to World War 2. Aside from passenger comfort and appeal, the DC-3 offered that most important of virtues, profitability, with the result that over 400 had been sold to airlines prior to late 1941. Original public domain image from Flickr