This paper describes the system concept for the Stratospheric Observatory for Infrared Astronomy (SOFIA), as developed by in-house (Ames Research Center) Phase A level studies of the Telescope System and Ground Support/Operations System, and by contracted studies of the Aircraft System performed by the Boeing Military Airplane Company. The SOFIA facility will be a 3-meter class optical/infrared/submillimeter telescope mounted in an open cavity in the forebody of a Boeing 747 aircraft, to be operational in 1992. It represents the next generation of Ames' existing airborne IR facilities, including the Kuiper Airborne Observatory (KAO), which is a 0.91 meter telescope flown on a Lockheed C-141 aircraft. The SOFIA telescope will be about 10 times more sensitive than the KAO, will have 3 times better angular resolution, and will be able to detect all of the far-infrared point sources discovered by the IRAS (Infrared Astronomical Satellite) survey in 1983. We first present an overview of the SOFIA Phase A Telescope System concept, including its major requirements and design attributes. The Telescope System consists of the Telescope Assembly (optical train and support structures) and the Consoles and Electronics Subsystem, which provides the system's command, control, displays and communications. The major requirements and concept for the Aircraft System are next described, including the cavity modification and its supporting subsystems such as the cavity doors and shear layer control devices. Finally, a brief description of the Ground Support/Operations System is provided, including the ground-based facilities and equipment needed to support the airborne observatory, in addition to an overview of the operational scenarios and organization.