The High Throughput X-Ray Spectroscopy Mission (XMM) is a `Cornerstone' Project in the ESA long-term Program for Space Science. The satellite observatory uses three grazing incidence mirror modules coupled to reflection grating spectrometers and X-ray CCD cameras. In order to achieve a large effective area, each XMM mirror module consists of 58 Wolter I mirrors which are nested in a coaxial and cofocal configuration. This high packing density requires the production and integration of very thin mirror shells with diameters ranging from 300 to 700 nm. In this paper, we first present the optomechanical design of an XMM mirror module with an emphasis on thermal control and straylight analysis. We then describe the X-ray test results of a mirror development model with an electro-optical breadboard of a CCD focal plane camera.