This paper focuses on the generation of synthetic models of water distribution networks (WDN). Models are widely used in many fields related with WDN planning and operation. Therefore, the main contribution of this work is to provide an automatic procedure to build models with the well-known EPANET tool in a manner that, with a small amount of input data and a few clicks, the user can build a network topology and assign suitable pipe diameters. For that purpose, a new application, called WaterNetGen, was designed and implemented as an extension to the EPANET software. WaterNetGen can be used to generate synthetic models of WDN, with several hundred nodes and pipes, within a few minutes. The sizing capability allows the selection of commercial diameters, such that the final network design satisfies certain user-defined design constraints, like minimum diameter, maximum velocity and minimum pressure. The total water demand is allocated to the pipes taking into account their length and a demand coefficient. The water demand of each pipe is then assigned to its start and end nodes and follows a specific demand pattern.