Unmanned Aircraft Systems (UAS) know a tremendous interest in many fields thanks to their wide range of applications. UAS are complex "system of systems" because, in addition to stringent safety and embedded constraints, they incorporate heterogeneous interacting sub-systems. Adequate development approaches and tools have to be provided to deal with this complexity. A literature review concerning this perspective shows a lack because current UAS simulation tools focus on limited or isolated development application. We propose a system-level simulation platform for the development of UAS. The proposed platform supports a component-based design for the UAS where each component is developed using a based-model methodology. A network-oriented architecture and object-oriented conception was adopted to design the platform. This allows the latter to be modular, flexible and integrate many UAS components simultaneously. A Controller Area Network (CAN) data bus is integrated to the platform. It will permit a "smooth" transition from the simulation phase to experimentation phase. To accelerate the platform design, a COTS flight simulator is integrated. This latter implements the dynamic, graphic and environmental model of the aircraft.