A crossbow with built in cocking device includes a manual crank and a built-in, removable motor gearbox assembly and power source. Rotational forces applied to a drive shaft rotate in a first direction, causing first and second string carriages to engage a bow string, moving the bow string towards a string catch. Once the string catch is latched to the bow string, a drive shaft is rotated in a second direction to move first and second string carriages to the a rest position.