A conceptual multidisciplinary design framework for High Altitude Long Endurance (HALE) aircraft is developed for solar-powered single and multiple-boom aircraft configurations. A defining feature of high-aspect ratio HALE vehicles is the tight coupling among various disciplines in particular aerodynamics and structures. A physics-based framework is required to fully exploit potential couplings that may result in significant mass savings. In order to quickly and accurately explore the design space, first-order physics are employed where possible and reliance on historical empirical data is minimized. Although the framework is useful to rapidly down-select potential configurations, sufficient engineering fidelity is also captured resulting in realistic preliminary designs enabling shorter engineering and development cycles. Low Reynolds number aerodynamics, composite structures, integrated vehicle performance (including solar energy utilization) and their interactions, are captured with an appropriate level of fidelity while maintaining computational efficiency. Several aspects of the framework are validated using higher-fidelity analysis tools. In this paper (Part I), optimization case studies for single and dual-boom configurations are discussed.