It had an uncommon appearance due to its combination of unusual design features, such as its twin boom tail unit, short hull, and trimotor engine configuration.

Shortly after the maiden flight of the first prototype (D-ARAK)[1] on 15 July 1937, identified instabilities necessitated a redesign of the hull and tail booms to improve both hydrodynamic and aerodynamic properties.

The first production-standard aircraft, designated BV 138A-1, performed its first flight during April 1940, by which point Germany was at war with multiple neighbouring nations.

Despite concerns as to its structural strength, which was improved over time, it was adapted into various roles, including as a radar-equipped anti-shipping aircraft, an austere troop transport, and as an airborne aerial countermeasure to naval mines.

[3] In terms of its general configuration, it had a relatively short hull, a high-mounted wing that incorporated not only a pair of engines but also floats, and twin boom tail unit.

[3] This stage of development was protracted, in part due to the need to address the original engine difficulties with an extensive redesign.

Several changes were made, particularly in the area of the vertical tail surfaces and the hull, on the second prototype after flight testing revealed both hydrodynamic and aerodynamic instability.

[5] During April 1940, the first production-standard aircraft, designated BV 138A-1, performed its first flight; shortly thereafter, it was rapidly introduced into Luftwaffe service in response to pressing wartime demands.

It had a relatively compact hull which, with its hydrodynamic step beneath and flat sides, earned it the nickname, "Fliegender Holzschuh" (the flying clog).

The booms of the twin tail unit, much like the smaller Focke-Wulf Fw 189 twin-engined reconnaissance monoplane, extended horizontally from the rear of the outer engine nacelles.

The first standardized version, BV 138B-1, was powered by three 880 PS (868 hp, 647 kW) Junkers Jumo 205D two-stroke, opposed-piston aircraft diesel engines.

[10] In addition to its use in the maritime reconnaissance role, several aircraft were specially adapted to sweep naval mines; designated BV 138MS, this role necessitated the deletion of all conventional armaments and the installation of a large (roughly 40 feet (12 m) diameter) dural hoop mounted horizontally under the wings, powered by an auxiliary motor generating an electric field which detonated magnetic mines.

[8] For naval reconnaissance, some aircraft carried FuG 200 Hohentwiel low-UHF band maritime search radar sets; this enabled the type to be effectively used to conduct anti-shipping missions.

[12] The BV 138 was tested with the Walter HWK 109-500 Starthilfe RATO jettisonable rocket pod, used in pairs, for shorter takeoff performance.

However, the wreck of one aircraft, sunk after the war in a British air show, was raised from the seabed of the Øresund Sound in 2000, and is on display at the Danish Technical Museum in Helsingør.

[17] In June 2013, a vessel from the Norwegian Geological Survey filmed a Blohm & Voss BV 138 at a depth of 35 m in Porsangerfjorden, Norway, not far from the WWII German seaplane harbour in Indre Billefjord.