The network-centric 'system-of-systems' concept popular in current defense programs has been viewed from a very functional perspective. However, the heart of such a system is going to be an embedded software infrastructure of unprecedented complexity, and the technology for developing and testing this software needs as much if not more immediate attention than the concept of operations for the envisioned applications. Such an embedded software system will need to be infinitely scalable, modular, verifiable, and distributed, yet satisfy the myriad hard real-time performance constraints imposed by each of perhaps many different device types and service demands. It is suggested here that the only path to a robust design methodology for such systems is with model-based design. Model-based embedded system design is the focus of the Model-Based Integration of Embedded Software (MoBIES) program, currently underway at the Defense Advanced Research Projects Agency (DARPA), managed by the author. This paper will motivate the model-based approach to large-scale embedded software design and explain how projects funded under MoBIES are contributing to the development of interoperable model-based design tool components. An application for such technology is provided in the context of digital flight control systems for aggressive aircraft maneuvers, which is the subject of another DARPA sponsored program, Software-Enabled Control (SEC).