This paper presents a software architecture for composing complete system-level simulations of mechatronic systems. The proposed architecture will provide the designer with the infrastructure to rapidly create simulations of alternative designs. The architecture promotes modularity and composability through the use of the design entity. Moreover, the architecture supports hierarchical modeling and provides the infrastructure to seamlessly integrate mechanics models with electronics and information technology models. Finally, the architecture facilitates distributed computing to take full advantage of the power of networked computers. This paper introduces the individual concepts of our architecture, and illustrates them in the design of a missile seeker.