Designed to compete against the DST/ DC-2/DC-3 series being developed by the Douglas company, the Lockheed 14 Super Electra failed, by reason of its smaller capacity, to present any significant competition. Of the same general configuration as the earlier Lockheed 10 Electra, it differed primarily by having a much deeper fuselage accommodating a maximum of 14 passengers, a mid-set wing, and introduced such advanced features as integral fuel tanks in the wing, Fowler-type trailing-edge flaps, fully-feathering propellers and, at a later stage of production, fixed wing slats. These improvements, combined with powerful engines and high wing loading, gave the Super Electra excellent performance but, by comparison with the important and larger-capacity Douglas DC-3, it was less efficient in operation, with the result that only 112 were built by Lockheed. First flown in prototype form on 29 July 1937 and certificated on 15 November 1937, initial deliveries were made shortly afterwards. By far the majority of the Super Electras were exported and, in addition, a total of 119 was licence-built in Japan for use by the Imperial Japanese Army. These, powered by 671kW Mitsubishi Ha-26-l radial engines, were designated Army Type LO Transport and were later allocated the Allied code name 'Thelma'.