The BK 117 was developed as a joint venture project between Germany's Messerschmitt-Bölkow-Blohm and Japan's Kawasaki for a successor to the highly successful Bo 105 light helicopter for both military and civil operations. The twin-engined BK 117 is capable of transporting 11 troops as well as additional cargo via rear clamshell doors, it also has significant combat capability in the form of a weapons load composed of machine guns, rockets, or HOT anti-tank missiles, and can carry defensive aids such as a radar-warning receiver and countermeasures launcher equipment. Other roles for the BK 117 include law enforcement, medical evacuation, offshore support and search and rescue (SAR) duties and it has been exported to Canada, Chile, Iraq, South Africa, the UAE, and license built in Indonesia. A militarized version also won the US Army's Light Utility Helicopter program for a replacement of the OH-58.

The BK 117 was first flown on 13 June 1979 and entered service in its initial civil version, the BK 177A-1. These were followed by the BK 117A-3 with a larger tail rotor and increased take-off weight, and the BK 117A-4 with significantly enhanced performance. A military version is known as the BK 117M and features a machine-gun turret under the fuselage, helmet-mounted sights, troop-carrying capability, and the ability to carry anti-tank and air-to-air missiles. Finally, a number of further upgrades has resulted in the BK 117B with an uprated powerplant and the BK 117C which eventually led to the redesignated EC 145 (BK 117C-2). The EC 145 differs in having a twin-turboshaft powerplant as well as a slightly redesigned fuselage, it won the US Army's LUH requirement and will enter US service as the UH-72 Lakota and is described separately.