A total of 297 BV 138s were built between 1938 and 1943. The aircraft was unusually powered by three engines, with one mounted high above the centerline driving a four-blade propeller, and one on each wing driving 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 aircraft diesel engines. Unusual were the aircraft's twin boom tail unit, and defensive gun emplacements at the bow (an enclosed gun turret mounting an MG 151/20 cannon, visually similar to that used on the C-3/U1 model of the Focke-Wulf Fw 200 Condor) and a fully open Scarff ring-like emplacement at the stern of the fuselage, as well as another open gun position behind the central engine. These features together produced the aircraft's ungainly appearance.

The first of the 227 standard service variant, BV 138 C-1, began service in March 1941. Although various versions of the aircraft carried a variety of armament, the standard included two 20 mm MG 151/20 cannons, one in a power-operated bow turret and one in a power-operated stern turret, up to three 7.92 mm MG 15 machine guns, and a 13 mm (.51 in) MG 131 machine gun in the aft center engine nacelle. It could carry 500 kg (1,100 lb) of bombs or depth charges or, in place of these, up to 10 passengers. Several were later fitted with FuG 200 Hohentwiel low-UHF band search radar for anti-shipping duties. Some were converted for minesweeper role. The BV 138 MS variant, with the "MS" suffix signifying Minensuch (German for mine-clearing, literally mine-search), carried a degaussing device, a hoop with the same diameter as the length of the fuselage and field-generating equipment, instead of weapons.
C1
Type V1 MS  4 seat mine sweeping aircraft
Engine 3 Jumo 205 C 3 Jumo 205 D
Dimensions Length 12,20 m , height  ,  span  , wing area   , Length  , height  ,  span  , wing area
Weights Empty , loaded  , max. take off weight   Empty , loaded 16000 kg , max. take off weight 
Performance Max.. speed  , cruising speed  , range , endurance  , service ceiling   , climb Max.. speed  , cruising speed  , range , endurance  , service ceiling   , climb
Armament 1 MG 131 in dorsal position
Version A1 B1 C1
Type 6 seat reconnaissamce flying boat
Engine 3 Jumo 205 C4 Outer Engines prop. VDM 3-bladed adjustable pitch dia. 3,30 m. Central Engine  first 3 bladed prop. dia 3,0 m later 4 blades 3 Jumo 205 D-1 Outer Engines prop. VDM 3-bladed adjustable pitch dia. 3,30 m. Central Engine  first 3 bladed prop. dia 3,0 m later 4 blades 3 Jumo 205 D-2 Outer Engines prop. VDM 3-bladed adjustable pitch dia. 3,30 m. Central Engine  first 3 bladed prop. dia 3,0 m later 4 blades
Dimensions Length  19,85 m, height 5,90 m (with prop. running 6,60 m) ,  span 26,94 m , wing area  112 m2 ,
Weights Empty 10800 kg, loaded 16200 kg
Fuel in 3 tanks in mid wing tube (armoured) 5310 l. 3 tanks unprotected in wng spar		 Empty 11355 kg, loaded  17400 kg Empty 11770 kg , loaded 17650 kg, with startrockets 18150 kg 
Performance Max.. speed 265 km/h at sea level , cruising speed 240 km/h , range 3930 km, service ceiling  3800 m Max.. speed 285 km/h at sea level , cruising speed 235 km/h , range 3880 km, service ceiling  4200 m Max.. speed 285 km/h at sea level , cruising speed 200 km/h , range 4300 km, service ceiling 5000 m 
Armament Front 1xMG204 or. 1x MG 15 ; Tail 1xMG204 or. 1xMG 15 ; dorsal 1xMG 15 : : 3x50 kg bombs or 2x FI.W.B. 150 Kg (Mines) Front 1x MG 151/20 (1000 rpg): tail 1x MG 151/20 (1000 rpg); dorsal SAL 1x MG 15 (1500 rpg) r. 1x MG 131 (900 rpg); : 3x 50 kg bombs or 2x FI.W.B. 150 kg (Mines) Front 1x MG 151/20 (1000 rpg): tail 1x MG 151/20 (1000 rpg); dorsal SAL 1x MG 15 (1500 rpg) r. 1x MG 131 (900 rpg); : 6x 50 kg bombs or 4x depth charges 150 kq orr 2x FuG 203 targetmarkers
Equipment Radio FuG X (Bordfunkgerät) with EiV (Eigenverständigung); Peil G V (Peil- und Zielfluggerät); FuBI 1 (Funklandegerät)
FuG 200 "Hohentwief (Schrffssuchgerät up to 100 km range) a few airctaft had FuG 213 "Lichtenstein S" (Schiffssuchgerät up to 30 km) a few had  FuG 216 "Neptun R" (Rückwärtswarnradar)