Model-Based Development (MBD) has become an automotive industry standard process in vehicle control systems development due to the potential to reduce development time and improve engineering quality. This has become even more important as control systems are becoming increasingly complex while development cycle timelines shorten. Toyota utilized Hardware-in-the-Loop Simulations (HILS) techniques when developing the latest Fuel Cell Hybrid Vehicle, FCHV-adv. These MBD techniques contributed to the overall development process, but applications were limited to verifying control specifications. It was recognized that if MBD could be utilized beyond its current role in the control system design process to include functional specification validation, specification quality could be improved while decreasing development time and cost. In order to expand and improve MBD for functional specification development, conventional Software-in-the-Loop Simulations (SILS) were adapted and implemented as part of a MBD-centered process. This paper describes the development and verification of a vehicle model for FCHV control and functional specification development within a SILS environment.