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The Flight Mirror Assembly (FMA) preliminary mechanical design for NASA's next major X-ray telescope mission, the International X-Ray Observatory (IXO), has been developed at NASA Goddard Space Flight Center (GSFC). The design addresses some unique engineering challenges presented by the unprecedented combination of high angular resolution and large effective area required to achieve the desired scientific objectives. To meet these requirements, the Wolter-I Soft X-Ray Telescope (SXT) optical design consists of about 14,000 0.4 mm thick glass mirror segments densely packed into a 3.4 m diameter FMA and supported with micron level accuracy and stability. Key engineering challenges addressed include ensuring positive stress margins for the glass segments with a high Factor of Safety, keeping the structure light enough to launch, providing a large effective area, and preventing unacceptable thermal distortion. Standard mechanical design techniques such as FEM modeling and optimization, integrated optomechanical analysis, and development testing were applied to this unique problem. The thin mirror segments are mounted into 60 intermediate wedge shaped structures called modules. Modules are kinematically mounted to the FMA primary structure which is optimized for minimum mass and obscuration of the clear aperture. The preliminary design demonstrates the feasibility of building and launching a large space-based SXT using slumped glass mirrors which meets the IXO effective area, mass, structural, and thermal requirements.