Originally conceived in 1934 as a very long-range reconnaissance flying-boat, the Blohm und Voss Ha 138 VI prototype was first flown on 15 July 1937 as a shoulder gull-wing flying-boat with twin tail booms and three Jumo 205C engines.
Directional stability and poor water handling characteristics in the prototype caused extensive redesign in the pre-production version, the Bv 138A-0, of which six were built with a considerably enlarged hull and an ungulled wing.
The first 25 production Bv 138A-1 aircraft flew in April 1940, taking a limited part in the invasion of Norway, and entered general service in western France late that year. Considerable structural strengthening was found to be necessary and this was incorporated in the Bv 138B-1, 14 of which emerged from the production line in December, and seven more in 1941, powered by 880-hp (656-kW) Jumo 205D engines. A new turret was introduced mounting a single MG 151 20-mm cannon forward of the pilot's cockpit and another in the rear of the hull. The Bv 138Bs were very active in 1941, particularly those based in Norway after the sailing of the first North Cape convoys. Trouble had been experienced with the Bv 138B-l's engines and propellers, however, and an improved version, the Bv 138C-1, in which all the previous problems were eliminated, began appearing in March 1941; 227 of this version were built before production was terminated midway through 1943. In this model the centre Jumo 205D drove a four-blade propeller and provision was made for increased bomb-loads. The most successful exponents of the Bv 138C were probably the crews of Küstenflieger-gruppe 406 based in northern Norway, this unit being responsible for much of the successful locating and shadowing of the North Cape convoys, particularly PQ 16 in April 1942. Some aircraft were equipped with FuG 200 Hohent-wiel search radar for anti-shipping duties, while in the transport role the Bv 138 could carry up to 10 passengers.
All versions could be fitted with two 500-kg (1,102-lb) thrust assisted take-off rockets, and a number of redundant Bv 138B-ÛS, redesignated Bv 138MS, were fitted with a large durai hoop energized by an auxiliary generator for magnetic mine clearance with the Mmensuchsgruppe.
Blohm und Voss Bv 138C-1
Type: five-crew long-range maritime reconnaissance flying-boat
Power-plant: three 880-hp (656-kW) Junkers Jumo 205D 12-cylinder inline diesel engines
Performance: maximum speed 285 km/h (177 mph) at 3000 m (9,845 ft); normal service ceiling 5000 m (16,405 ft); maximum range 4300 km (2,672 miles)
Weights: empty 11780 kg (25,970 lb); maximum take-off 17670 kg (38,995 lb)
Dimensions: span 26.94 m (88 ft 4 in); length 19.85 m (65 ft 1 Vz in); height 5.90 m(19 ft4!/4 in); wing area 112.00 m2(l,205,56 sg ft)
Armament: one 20-mm MG 151 cannon each in bow and stern turrets, one 13- mm(0,51-in)MG 131 machine-gun in open position aft of central engine, and one 7.92-mm (031 -m) MG 15 gun in hatch of starboard side of hull, plus up to four 150-kg (331-lb) depth charges or equivalent weight of bombs on six racks