The design of the Lockheed L-188 Electra began in 1954, and 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. Lockheed built a total of 170 Electras when production ended unexpectedly early due to passenger loss of confidence in the type after two had disintegrated in flight. By the time Lockheed had incorporated remedial modifications, customer airlines were interested in turbojet- rather than turboprop-powered aircraft. Lockheed Aircraft Service converted many of them for convertible passenger/cargo or all-cargo use.