In the mid-1980s, MBB Helicopters launched a new design for a light, medium twin. A technology demonstrator for this helicopter, originally known as the Bo 108, first flew in 1988. By 1991, MBB Helicopters had become part of Eurocopter and the decision was made that the Bo 108 would replace the Bo 105. Over the course of the next several years, the original Rolls-Royce 250-C20R engines were replaced with more powerful engines, the cabin was lengthened and widened to allow a total of seven seats, the tail rotor was replaced by a Fenestron-shrouded tail rotor, and it was given the EC135 designation. In addition, the decision was made to offer not only Turbomeca engines but also Pratt & Whitney Canada engines.
The EC135 is certificated to both FAR and JAR 27 standards with Category A and IFR capability. The drivetrain for this helicopter uses either two Turbomeca Arrius 2B1 engines or two Pratt & Whitney Canada PW 206B engines. With the Turbomeca engines, the designation is EC135T1 and with the Pratt & Whitney engines, it is EC135P1. The helicopter has a four-bladed rigid rotor head with flexible composite arms that connect the rotor head to the composite blades and an 11-bladed, composite Fenestron-shrouded tail rotor. The fuselage is constructed largely of composites with a cabin that has, like its predecessor the Bo 105, clamshell doors aft. The standard seating configuration has two seats up front; three passenger seats facing aft; and two passengers in the back, all in energy attenuating seats. Alternatively, two stretchers loaded through the clamshell door and two attendants can be accommodated in addition to the pilot.
In 1994, the EC135 pre-production prototypes made their first flight and were certificated in 1996. First deliveries were made the same year.