This paper investigates the fuel saving potential of a series hybrid military truck using a combined battery pack design and powertrain supervisory control optimization strategy. The design optimization refers to the sizing of the Lithium-ion battery pack in the hybridized configuration. On the other hand, the powertrain supervisory control optimization finds the most efficient way to split power demands between the battery pack and the engine. Most of the previous literatures implement them separately. Combining the sizing and optimal control problem in a single optimization routine might produce better fuel economy in a more efficient manner. This study proposes a novel unified framework to couple Genetic Algorithm (GA) with Pontryagin’s Minimum Principle (PMP) to determine the battery pack sizing and the power split control sequence simultaneously. As GA and PMP are global optimization methodologies under appropriate conditions, our results can be regarded as the benchmark for the development of this hybrid military truck. Military drive cycles were further applied under the combined optimization framework to leverage the impacts of different driving conditions for a final battery pack size and control solution. This study serves to supplement the understandings on the optimization of hybrid military vehicles and contribute to successful fielding of them in near future.