The design of the Lockheed L-188 Electra began in 1954, and in the following year the company received a launching order from American Airlines. The prototype, first flown on 6 December 1957, was a low-wing monoplane of conventional configuration with retractable tricycle landing gear and powered by four Allison 501D-13, 501D-13A or 501D-15 turboprop engines. Standard accommodation was for 66 to 80 passengers, but a high-density arrangement was available optionally to seat 98. Built initially as the L-188 A the Electra became available also as the longer-range L-188C with increased fuel capacity and operating at a higher gross weight. A total of 170 had been built when production ended unexpectedly early as a result of passenger loss of confidence in the type after two had disintegrated in flight, and by the time remedial modifications had been, incorporated customer airlines were interested in turbojet- rather than turboprop-powered aircraft. Many of them were converted by Lockheed Aircraft Service for convertible passenger/cargo or all-cargo use.