Scramjet propulsion is a hypersonic airbreathing technology that offers the great potential for economical and flexible access to space and high-speed atmospheric transport. This paper presents the results and physical insight obtained in a design optimization study conducted for axisymmetric scramjet engines. A single-objective optimization has been performed to maximize the performance of a full flow-path scramjet configuration in the presence of nonuniform upstream fuel injection represented by a set of geometric parameters and injection scaling factors. Use is made of a state-of-the-art design methodology coupling a high-fidelity CFD code with an advanced optimization capability based on evolutionary algorithms assisted by surrogate modeling. The flowfields have been investigated to identify key factors and gain insight into underlying physics particularly in comparison with premixed fuel/air flowfields. The optimum flowfield with fuel injection is characterized by a mixed supersonic/subsonic flow regime similar to a transitional mode. The optimum design has been achieved by maximizing the nozzle thrust while reducing skin friction drag by boundary-layer combustion, although it represents a sensitive flowfield due to the delicate mechanism upon which it relies.