The H135 is a twin-engine rotorcraft. It can be alternatively powered by a pair of Turbomeca Arrius 2B or Pratt & Whitney Canada PW206B engines, dependent on customer’s preference (which gives either a T or a P, respectively, in the variant name). The main rotor is of a four-bladed, hingeless fiber-composite design; progressive improvements to the main rotor have increased its performance and reduced maintenance costs since the type’s introduction. The EC135 holds the distinction of being the quietest helicopter in its class, featuring an anti-resonance isolation system to dampen vibration from the main rotor. The type’s fenestron anti-torque device can be actively regulated via a HI NR rotor optimization mode, which provides for greater controllability during higher weight take-off and landings. It is capable of performing Category A operations throughout its full flight envelope.

The EC135 can be equipped with either a conventional flight deck or the Avionique Novelle glass cockpit – the latter allows for single pilot instrument flight rules operation. The glass cockpit is equipped with several liquid-crystal displays, including two Sextant SMD45 displays and a central panel display. The main avionics suite is supplied by Thales Group; the EC135 can be outfitted with various avionics suites from manufacturers such as Russian firm Transas Aviation and British firm Britannia 2000.

The newer H135 model can be equipped with a four-axis autopilot, which is included as part of the Helionix avionics suite; this suite provides the H135 a greater level of commonality with several other Airbus Helicopters-produced rotorcraft including the H145, H160 and H175. Earlier versions of the EC135 were equipped with a three-axis autopilot with integrated stability augmentation; featuring a First Limit Indicator (FLI), simplifying engine and torque monitoring. Cockpit touch screens can be optionally installed.