A takeoff assist device for an air vehicle, such as an unmanned air vehicle is provided. The device features an electric motor and is much less complex and lighter than takeoff assist devices heretofore known. A body member is shaped to fit to a portion of the air vehicle. The body member defines a housing in which the majority of the components are housed, including the electric motor, a motor controller, an electrical power source and a main controller. A propeller is attached to a shaft of the electric motor outside of the housing and rotates with a shaft of the electric motor. In addition, there is a latch mechanism that removeably attaches the body member to the host air vehicle for takeoff, and then is controlled to release from the air vehicle after it is airborne at a desired airspeed and altitude. The electric motor-driven takeoff assist device can launch an air vehicle with minimal auxiliary equipment and can be remotely controlled to launch from a site at which a takeoff ready vehicle can be left for extended periods of time.