The appearance of the BV 138 was unique, in its combination of unusual design features, such as: twin boom tail unit, trimotor configuration, and fuselage slung beneath the wings. One nickname, "the Flying Clog" was derived from the shape of the slab-sided hull unit. These features together produced the aircraft's ungainly appearance, but inspired a certain affection among its crew and mechanics.

Three piston engines were used. The central engine was mounted above the wing, driving a four-blade propeller, while the wing engines were lower, with three-blade propellers. The pre-production prototypes and the BV 138 A-01 to BV 138 A-06, were powered by various makes of engines ranging from 485–746 kW (650–1,000 hp). The first standardized version, BV 138 B-1, was powered by three 880 PS (868 hp, 647 kW) Junkers Jumo 205D two-stroke, opposed-piston aircraft diesel engines. The engine cowlings also had an atypical appearance, due to the unique nature of the vertical orientation of the six-cylinder opposed-piston Jumo 205 diesel engines, and resembled the cowlings of 4 or 6-cylinder inverted inline engines found on smaller civil and utility aircraft from the Jumo 205's propshaft placement, emerging forward at the uppermost front end of the powerplant.

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.

For hydrodynamic reasons, the hull featured a distinct "turn-down", or "beak" at the stern.

Two enclosed, powered gun turrets, each mounting a single MG 151/20 autocannon, were located prominently at the bow and stern. A third, fully open Scarff ring-like emplacement, behind the central engine and both above and forward of the rear turret, mounted a 13 mm MG 131 heavy machine gun covered fields of fire obstructed from the other turrets by the horizontal stabilizer.

Sources:
Gunston, Bill & Wood, Tony - Hitler's Luftwaffe, 1977, Salamander Books Ltd., London
Wikipedia - Bv 138
Donald, David - The Complete Encyclopedia Of World Aircraft, 1997, Brown Packaging Books Ltd., London