This paper describes the mechanical design of the primary mirror that images the synchrotron light extracted from the High Energy Ring of the PEP-II B Factory. During operation, the water-cooled GlidCop mirror is subjected to a heat flux in excess of 2000 W/cm^2, and the imaging surface is plated with Electroless Nickel to improve its optical characteristics. Calculated mechanical distortions and stresses experienced by the mirror during off-axis and normal operation will be presented.