[citation needed] Development of the Ha 139 started in 1935 to fulfil a specification issued by the German flag carrier Deutsche Luft Hansa.

Built to serve on the airline's long distance routes, it was equipped with four hydraulically-actuated landing flaps, was powered by four Junkers Jumo 205C diesel engines, and was suitable for catapult-assisted takeoffs.

Continued development of the aircraft led to a land-based version, the Blohm & Voss BV 142,[1] while a proposed reconnaissance/bomber variant of the Ha 139 for the Luftwaffe was never realised.

[8] During March 1937, Deutsche Luft Hansa received the first aircraft, Ha 139 V1; shortly thereafter, the airline commenced trials in the North Atlantic between the Azores and New York using both V1 and V2 and a pair of seaplane depot ships.

[6] During these flights, the aircraft routinely achieved average cruising speed just shy of the specification, however, the discovery of several faults led to the trials being temporarily halted in November 1937 while remedial work was undertaken.

These modifications included replacement rudders and fins with counterparts possessing greater surface area (to counteract directional instability) and the elimination of all inner-engine radiators in favour of an underwing arrangement.

[6] The type was flown by Deutsche Luft Hansa on various transatlantic routes, predominantly in the South Atlantic between Bathurst, The Gambia and Natal, Brazil.

[10] Introduced to service during the Norwegian campaign, the militarised Ha 139s were typically used to perform transport (of both troops and freight), reconnaissance and minesweeping sorties over the Baltic Sea.