Called the "flying sabot" because it was boat-shaped, the BV 138, despite prolonged development and testing and many technical problems that lasted several years, was the German flying boat that would be built in the greatest numbers. By 1934, 3 aircraft companies had been requested by the German Air Ministry to submit designs for a long-range reconnaissance flying boat that would be capable of operating on the high seas. HFB (Blohm & Voss) was then awarded the contract and in the summer of 1936 the HA 138 VI made its maiden flight. The testing of the V2 revealed that further design modifications to the airframe and landing gear were needed, so that the BV 138 A-01 did not achieve a satisfactory flying performance until the summer of 1939. During the Norwegian campaign in April 1940, three aircraft of the nil series were successfully used as transports between Norderney and Narvik. In addition to the production of 25 BV 138-A-1s and 20 BV 138 B-1s, from 1941 onwards, a total of 228 examples of the C-1 range were produced by Blohm & Voss and the Weser Flugzeugbau GmbH. In July 1940 after only a few hours operation the sea reconnaissance units incurred serious damage to the engines and airframes, however despite many improvements these faults could never be totally eliminated. The trial mine sweeper built in November 1942 produced unsatisfactory results, so the testing was discontinued after only a few flights. The production of the BV 138 ceased in December 1943.