Flight Simulator 5 (FS5), released in 1993, was the last DOS-only release in the popular Microsoft Flight Simulator series which began in 1982.
FS5 marked a significant change in that it was the first version of the series to use textures. While this allowed FS5 to achieve a much higher degree of realism than the previous flat-shaded simulators, it also resulted in all add-on scenery and aircraft for the previous versions obsolete.
Other noticeable improvements included the use of digital audio for sound effects, custom cockpits for each aircraft (previous versions had a generic cockpit that was slightly modified to fit various aircraft), and overall improved graphics.
FS5 also featured:
Expanded bundled scenery (now including parts of Europe)
Improvements to: modeling of the included aircraft, weather system's realism and artificial intelligence
Source: Wikipedia, "History of Microsoft Flight Simulator", available under the CC-BY-SA License.