It was originally developed by the Hamburger Flugzeubau and the prototypes were initially denominated as Ha.138 V1, and Ha.138 V2 which were completed on 15th July 1937 and August 1937 respectively, although they weren't tested until November that year.
Their appearance was unique with its combination of unusual design features with its twin boom tail unit, short fuselage and trimotor engine configuration. The short hull with its hydrodynamic step beneath and flat sides, earned it the nickname "Fliegender Holzschuh" - the flying clog-.
The three prototypes were powered, by three Junkers Jumo 205C which yielded a power of 592hp each and, unlike production versions, like the Bv.138A, they drove three-bladed propellers each.
Eventually it came to be the most numerous aircraft manufactured by the Blohm & Voss which was a shipbuilding company and founded its aircraft subsidiary, the Hamburger Flugzeubau on 4th July 1933 under the control of Walter Blohm. Their first aircraft, the Ha.135 was a conventional biplane trainer which was designed by Reinhold Mewes, but he was soon replaced by Dr. Ing. Richard Vogt who was responsible for many of Blohm & Voss designs. From 1933 to 1937 their aircraft carried the Ha. designation, but that changed to Bv. in 1937. As that changement took place during the development of the Bv.138, the prototypes were known as Ha.138 and the production aircraft as Bv.138.
It was the first flying boat to be produced by Blohm & Voss which previously had examined a series of possible designs before project P.12 was selected as the basis for a long-range reconnaissance aircraft. The original design had shorter hull and a high mounted wing with twin engines, booms and floats attached to the wings.
A mock-up was built in early 1935 and Blohm & Voss was awarded by the Reich Air Ministry with a contract to produce three prototypes, each to be powered by a different type of an engine that could reach 1000hp. All three of the engines ran into design problems and Vogt was forced to redesign the aircraft using the aforementioned trio of Junkers Jumo 205C engines with the third engine mounted above the centre of the wing. These engines, in this configuration, produced almost the same amount of power as two 1000hp engines, but the third one produced extra weight and drag.
Those first prototypes had short hulls, smaller tail surfaces and narrow booms. Testing revealed that they were unstable both in the air and in the water, so a complete redesign was imposed, which began in 1938.
Sources:
1. https://en.wikipedia.org/wiki/Blohm_%26_Voss_BV_138
2. https://en.wikipedia.org/wiki/Hamburger_Flugzeugbau
3. http://www.historyofwar.org/articles/weapons_blohm_und_voss_bv_138.html
4. https://www.valka.cz/topic/view/59211/Blohm-and-amp-Voss-BV-138-A-1
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