A large area position sensitive X-ray detector has been developed for use in the coded mask imaging X-ray spectrometer (COMIS) aboard the USSR research module KVANT. The module was launched on March 31, 1987. The detector, having a sensitive area of 256 × 256 mm 2, is a sealed multiwire proportional counter filled with Xe gas. It covers the photon energy range 2 to 25 keV. At an energy of 6 keV the achieved position resolution is better than 0.6 mm in the direction parallel to the anode wires. The energy resolution is about 18% at 6 keV. The paper gives an overview of the development considerations for optimizing performance with respect to linearity and resolution in energy and position response. Results of performance verification are presented.