The Stratospheric Observatory for Infrared Astronomy (SOFIA) will enable unique astronomical observations from visible to millimeter wavelengths. AIRES, a long-slit spectrograph with a mid-infrared slit viewing camera, would enable spectral imaging of gas-phase spectral features between 17 and 210 μm with resolving powers from ~60,000 to 5000. The Cryogenic Grating Spectrometer (CGS: AIRES' predecessor) which was flown on NASA's Kuiper Airborne Observatory (KAO) for 13 years, demonstrated the importance of this wavelength range. A 1997 proposal to develop AIRES was selected as the highest-ranked of 19 U.S. competitors for first-generation SOFIA science instruments. Funding was terminated in 2001 due to budget problems associated with an original under estimate and the advent of full cost accounting in NASA. Here we summarize AIRES' expected performance, its science potential, its status, and lessons learned. Highlighted are three successfully accomplished major technical developments: the world's largest monolithic cryogenic grating, cryogenic multiplexing amplifiers for far-infrared germanium photoconductor detectors, and an optical/mechanical design in a package suitable for installation on SOFIA. We show that AIRES would fill a unique role among other spectroscopic capabilities foreseen for space-borne missions. AIRES' capabilities remain a high but unfilled priority for SOFIA, and for the science community in general.