The aim in developing the helicopter was to continue to use as many proven components of the predecessor model Bölkow Bo 105 as possible, but at the same time to keep up with the state of the art and to create more space in the interior. Especially in rescue operations, the cramped conditions in the Bo 105 had proven to be a hindrance at times. The cooperation agreement between MBB and KHI was signed by Ludwig Bölkow and Teruaki Yamada on 25 February 1977. It provided for MBB to develop the new main rotor, including the associated control components, and KHI to develop the airframe and main gearbox. The development costs of an estimated US$100 million were split equally. On 13 June 1979, the maiden flight of prototype P2 of the BK 117 took place in Ottobrunn. This prototype still exists today and can be seen in the Bückeburg Helicopter Museum.

MBB later merged with the French company Aérospatiale as part of Daimler-Benz Aerospace (DASA) to form the new company Eurocopter, whose product range still includes the BK 117 today – now in the BK 117-C2 variant known as the EC145.The BK117 D2 model was sold under the product name EC145 T2 until March 2015. Since then, the latest T2 variant has been sold under the product name H145. In 2019, an upgrade of the rotor system was introduced. The previously used 4-blade system with titanium head will be replaced by a headless 5-blade system. The ATR (advanced technology rotor) system is said to result in less vibration, shorter blade length, lower empty weight and higher payload compared to its predecessor. It allows the blades to be folded for transport and facilitates baling. This version will be marketed under the product name BK117 D3.

The BK 117 is mainly used in air rescue and intensive care transport missions or as a police helicopter. Equipment packages from the manufacturer or third-party suppliers, such as a winch, are available for this purpose. This also enables the helicopter to perform SAR services in terrain that is difficult to access. In Germany, therefore, the ADAC air rescue service and the DRF Luftrettung have the largest contingents; in Spain, the Guardia Civil; in France, the Gendarmerie, among others, is equipped with the EC145. In some countries it was or is still in military use, for example in Iraq and South Africa or the United States (see UH-145).