The Cosmic Unresolved X-ray Background Instrument using CCDs (CUBIC) is designed to obtain spectral observations of the Diffuse X-ray Background (DXRB) with moderate spectral resolution (E/ΔE approximately 10 - 60) over the energy range 0.2 - 10 keV using mechanically collimated CCDs. It will be launched on the NASA/Argentine minisat SAC-B in December 1994. At this time, it is the only planned satellite payload devoted to the study of the spectrum of the DXRB. Observations will consist of 1 - 2 day pointed exposures of each target direction, resulting in a series of high quality spectra. Over the anticipated 3 year lifetime of the satellite, CUBIC will be able to study up to 50% of the sky with 5° × 5× spatial resolution for the subkilovolt Galactic diffuse background, and with 10° × 10° spatial resolution for the extragalactic diffuse background above 2 keV. CUBIC will obtain high quality non-dispersive spectra of soft X-ray emission from the interstellar medium, supernova remnants, and some bright sources, and will make a sensitive search for line emission or other features in the extragalactic cosmic X-ray background from 2 - 10 keV.