With the increasing complexity of systems and the pressing need for rapid development, there is a growing emphasis on creating inexpensive, flexible research testbeds for exploring system concepts and implementation options. Testbeds are platforms that provide the computational environment for system development which can be virtual (simulation) only, hardware only, or hybrid (simulation-hardware). The ability to deploy systems after studying their behavior under various conditions and with different use cases can dramatically reduce implementation risks. This is the rationale for modeling and simulation testbeds. However, often time testbeds are created with a focus on a specific application and without any consideration given to testbed flexibility and extendibility. Therefore, testbeds built for specific purposes have limited reusability and often end up being concept demonstrators or point solutions. A research testbed needs requisite flexibility, scalability, and extensibility so components can be readily added or removed as needed. This paper discusses the development of a structured methodology for developing a testbed for intelligent airborne and ground-based unmanned systems.