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.
In 1995, Flight Simulator 5.1 was introduced. It was the first version of the series to be released on CD-ROM. Changes included the ability to handle scenery libraries (including wide use of satellite imagery), faster performance and a barrage of true-to-life weather effects: storms, 3D clouds and fog.
Fans of the program generated a huge amount of custom "freeware" was created for FS5.1. Utility programs from Microsoft and third-parties allowed new aircraft, cockpits and airports to be created and added.
Source: Wikipedia, "History of Microsoft Flight Simulator", available under the CC-BY-SA License.