The Next Generation Air Transportation System (NextGen) brings significant advancements to the current management of the National Airspace (NAS). These fundamental changes have significant implications for safety and security, which, in turn, require new, more flexible techniques for the verification and validation of complex, software-intensive systems and systems of systems. To address this need, Barron Associates will develop a demonstration sense-and- avoid application, representative of the kinds of new systems that are possible in NextGen, and a safety case arguing that it is safe to operate in the NAS. The safety case will rely on run-time assurance and formal methods as evidence to support its claims. Run-time assurance continuously monitors system-level safety properties for impending violations to diagnose software faults and allows a simpler, high-criticality reversionary function to provide assurance for a more complex software function; formal methods provide strong design-time assurance of correctness for software that must operate at the highest levels of criticality. A safety-case-based approach citing these two strategies as evidence offers significant cost savings for similar or higher levels of assurance as compared to traditional, process-based approaches.