A irline profits critically depend on the nature and efficiency of service they provide, and accrue from a complex planning process involving schedule design, fleet assignment, aircraft routing, and crew scheduling, which are interrelated to each other within the overall system. We propose in this paper a model that integrates certain aspects of the schedule design, fleet assignment, and aircraft-routing processes, while considering flight retiming and demand recapture issues, along with optional legs, itinerary-based demands, and multiple fare classes. Maintenance routing decisions, as well as through-flight opportunities, are additionally incorporated in our model, and we apply the reformulation-linearization technique to reduce its complexity while introducing hierarchical symmetry-breaking constraints, along with other classes of valid inequalities, to enhance its solvability. A Benders' decomposition-based method is designed to handle the resulting large-scale model formulation. Computational results using real data obtained from United Airlines are presented to demonstrate the potential profitability in applying the proposed approach.