Fuel cells are one of the promising technical solutions for the first generation of sustainable aircraft. Currently, there are several shortcomings in the literature related to the conceptual and preliminary sizing of the propulsion systems of such aircraft: not only is there a lack of general methods, but also, much less effort is put into the estimation of the system’s weight, as compared to its performance. In this thesis project, a set of preliminary sizing methods was developed for the propulsion systems of CS-25 proton-exchange membrane (PEM) FC aircraft. These methods were then integrated into the Initiator, an aircraft sizing environment. With this approach, a complete preliminary sizing procedure of a regional turboprop aircraft was successfully performed. Finally, sensitivity studies were carried out in order to demonstrate the advantages of this modelling approach.