The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a proposed 3-meter class telescope in a Boeing 747 aircraft, anticipated as a joint development by NASA and West German Science Ministry. SOFIA would have the capability to make astronomical observations over a wavelength range from 0.3 microns to 1.6mm. The concept is based on the design and 13 years of experience in the Kuiper Airborne Observatory (KAO), a lockheed C-141 jet transport with a 0.9-meter diameter telescope, which SOFIA would replace. Relative to the KAO the larger telescope on SOFIA would provide a factor of 10 improvement in sensitivity for compact sources and a factor of 3 improvement in (diffraction limited) angular resolution at wavelengths beyond 30 microns. In addition, SOFIA will retain the major features of the KAO which have made the airborne astronomy program so successful. Among these are: continuous in-flight access to focal plane instruments while flying at or above 41,000ft altitude; pointing stability of 0.2 arc seconds; mobility and scheduling flexibility to accommodate targets of opportunity such as comets, eclipses, occultations, and novae. In this paper we present the scientific background, the scientific potential, a comparison with other astronomy missions, and the overall justification for an expanded airborne observatory.