Numerous research works, numerical simulations and real experiments have been dedicated to the use of Unmanned Aerial Vehicles (UAV) before, during and after wildfire occurrences, for multiple purposes including terrain and vegetation mapping for fire risk assessment and prevention, surveillance, fire detection and monitoring, gathering data for a human decision-maker, assisting search and rescue operations, and burnt area mapping and post-disaster evaluation. However, very few works deal with the firefighting process using UAVs and most of these consider only simulation and planning methods but not actual concept implementation and validation. This work describes the design, construction and test of a novel light-weight tethered UAV with mixed multi-rotor and water jet propulsion for forest fire fighting. The challenges in conceiving such system are presented and discussed. This analysis leads to a set of requirements and specifications for the system. A full-scale realization of the UAV is presented and tested in the field in a simulated fire drill. It is demonstrated that taking advantage of the water jet propulsion combined with a multi-rotor propulsion contributes to increase the flight endurance.