The BK-117 medium-weight multipurpose helicopter programme was given the go-ahead on 25 February 1977 with the signing of an agreement between the German consortium Messerschmitt-Bolkow-Blohm and the Japanese company Kawasaki Heavy Industries, following more than two years' negotiation. Both partners could boast considerable experience in the rotary wing sector and the new aircraft, which was an eight-ten-seater, was intended to replace two projects studied independently by the two companies: the German Bo.107 and the Japanese KH-7. Joint development costs were to be divided equally, the financing being guaranteed by the respective governments. MBB was to be responsible for the main rotor and tail rotor, tail boom, empennage, hydraulic system and controls, while Kawasaki was to develop the landing gear, fuselage, transmission and other minor components.

The original programme was based on four prototypes, two to be built by MBB in Munich and two by Kawasaki in Gifu, all to be completed by mid 1979. In each case, one of the prototypes was intended for flight testing and the other for static tests. However although the German company succeeded in completing its two models by the end of 1979, only one of the Japanese prototypes was ready on time.

The relationship of the BK-117 to the Bo.105 is evident in many respects. Firstly, a fair percentage of parts and systems are identical to those of the German helicopter. The hydraulic system is based on the original Bo.105 version and even the rotor is taken from the Bo.105, suitably enlarged to match the demands of the bigger and heavier BK-117. The four-blade rotor is of the rigid type with a titanium hub and reinforced fiberglass blades. The transmission is, of course, derived from the Japanese KH-7 project: a seven-ten-seat helicopter which was to have been fitted with two 590shp Lycoming turbine engines. The BK-117 in fact also has a pair of Avco-Lycoming LTS-101-650B-1 engines, delivering 600shp on take-off and 550shp maximum continuous power. The fuel tanks, with a total capacity of 605 liters, are housed in the lower part of the fuselage.

The executive version of the BK-117 carries a pilot and five passengers, but given the volume of 3.22m³ there is ample room for nine passengers in the high density version or in those used for commuter and offshore services to oil platforms. In the latter roles, it can carry a substantial payload, and even bulky goods can be easily loaded through two large sliding doors, one on either side of the fuselage. Behind the passenger cabin is a large baggage hold with a capacity of 1.34m³, which is reached through two hinged doors at the rear of the fuselage. Alternatively, the helicopter can be equipped to carry four stretchers plus two medical attendants; other roles include fire-fighting, search and rescue operations, or a cargo hook can be fitted to the cabin floor for external lift work.

The first German prototype flew on 13 June 1979 and the third (Japanese) aircraft on 10 August of that year. Production was initiated almost immediately and a year later, more than 100 BK-117s had been ordered. By the beginning of 1982, the BK-117 prototypes had logged more than 750 flying hours and type approval by the German Federal Authorities followed shortly afterwards. Half of the 130 aircraft ordered by February 1982 were for customers in the United States, where deliveries began in early 1983.

G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984

Following the signature of an agreement in February 1977, MBB and Kawasaki in Japan initiated the joint development of a new twin-turbine utility helicopter, suitable for both civil and military use, designated MBB/Kawasaki BK 117 With an airframe structure very similar to that of the MBB 105, the BK 117 combines the MBB BO 105's rigid rotor with a transmission developed by Kawasaki and introduces as powerplant two Avco Lycoming LTS 101-650B-1 turboshaft engines. It provides accommodation for a pilot and seven passengers as standard, or a pilot and up to six passengers in an executive layout. The BK 117 can be equipped for use in roles such as cargo transport, firefighting, law enforcement, medical evacuation (with pilot, one or two stretchers and two medical attendants/sitting casualties), offshore support, and SAR. German and Japanese prototypes flew for the first time on 13 June 1979 (D-HBKA) and 10 August 1979 (JQ0003) respectively. Kawasaki was first to fly a production aircraft (JQ1001) on 24 December 1981; MBB followed with D-HBKC on 23 April 1982, this machine being the first production aircraft to be delivered to a customer, in early 1983.

In addition to being built in Germany (at Donauworth) and Japan (Gifu), an agreement was concluded in 1982 for the type to be built in Indonesia under licence by IPT Nurtanio as the NBK-117. By January 1990 more than 250 BK 117s had been delivered worldwide, including 36 by Kawasaki, the standard aircraft now being the BK 117B-1. Since April 1990 a BK 117 engine testbed has been flying equipped with Turbomeca Arriel turboshafts in an effort to offer customers an alternative engine, and certification was scheduled for 1992. On 1 September 1991, MBB transferred its Helicopter Division to Eurocopter Hubschrauber GmbH. This was later integrated with Aerospatiale's helicopter interests into the Paris-based Franco-German Eurocopter Holdings, placing the NH-90, BO 105, BO 108 and BK 117 into a world class helicopter grouping.

VARIANTS

BK 117A-1: initial production version with LTS 101-650B-1 engines

BK 117A-3: certificated in March 1985 with larger tail rotor fitted with twisted blades and take-off weight increased to 3200kg

BK 117A-4: certificated in July 1986 with increased transmission limits at take-off power, improved tail rotorhead, and extra internal fuel (on German aircraft), all giving enhanced performance

BK 117 B-1: fitted with more-powerful LTS 101-750B-1 engines to provide further increased performance and 140kg more payload; certificated in 1987

BK 117M: military version of A-1 proposed by MBB in 1985, and flying since 1988; fitted with taller skids, a Lucas turret mounted under the fuselage houses a Browning 12.7mm automatic machine-gun and 450 rounds of ammunition, controlled by a helmet-mouted sight; outrigger pylons can carry up to eight HOT II or TOW antitank missiles, air-to-air missiles, rocket-pods, or forward-firing cannons; a doorway gunners position with a 12.7mm gun can also be installed, or 11 troops can be carried

D.Donald "The Complete Encyclopedia of World Aircraft", 1997

- MBB abandoned its armed military BK 117A-3M, but 'civilian' machines have been sold to military customers.

- Kawasaki flew the first production BK 117 on 24 December 1981.

- Several features of the BO 105 were retained or modified in the BK 117.

- MBB abandoned its BO 107 design and Kawasaki its KH-7 project in favour of joint BK 117 production.

- The first aircraft from the MBB production line flew on 23 April 1982.

- Germany's ministry of defence used one BK 117 as a composites testing aircraft.