The complexity of modern platform systems (e.g., commercial aircraft, spacecraft, and military vehicles) exponentially increases as we add new and advanced capabilities needed to sustain market and military leadership. Modern aircraft are aerodynamically unstable and controlled by hundreds of connected microcontrollers. Spacecraft dock at the International Space Station using artificial intelligence controllers, where humans are the 'backup system.' Military systems use cyber-assured autonomy and cyber-assured fire control on the battlefield. In each of these cases, new and significant engineering challenges arise. • Complexity Management - System design under the 'constraint of complexity' frequently leads to product cost overruns and delivery delays. • Dynamic over Statics - Current Model-Based Engineering (MBE) tools and techniques perform well for representing low-complexity systems with low interaction among design elements. However, they do not scale well for representing complex systems having high interaction among design elements. • Artifact Integrity - Most MBE methodologies do not appropriately integrate artifacts engineering disciplines (e.g., systems engineering, hardware engineering, software engineering, and test engineering) into an authoritative source of truth. Discrepancies in design artifacts may lead to undetected design flaws that leak into product manufacturing and end-user operations. Our analysis of MBE successes and failures from 2000 to 2020 suggests that we are at an inflection point where future design challenges may exceed the design capabilities provided by today's MBE techniques. This paper describes operational challenges and associated requirements leading to the development of BAE Systems' Model-Based Agile Engineering (MBAE) methodology. We present five innovations that provide the underpinnings for our MBAE implementation framework. We conclude with a brief description of a recent success story at BAE Systems and recommendations for the next steps in MBAE capability development.