The Douglas DC-1 is perhaps one of the most consequential types of passenger aircraft to enter service. Only one prototype was produced and served Transcontinental & Western Air (later Trans World Airlines or TWA) and two Spanish airlines (including Iberia). The DC-1 was the first model of the famous DC aircraft series, and Douglas used the DC-1 as a basis to develop its DC-2 variant and later, the hugely successful Douglas DC-3. The DC-3 is considered one of the most successful aircraft in the history of aviation.