The E-6 was developed in the 1980s and first flew on February 19, 1987. It entered service two years later, replacing the US Navy's EC-130 fleet. The E-6 was known as Hermes until 1991, when it was renamed Mercury. It is used as an airborne command post for the United States Strategic Command, instructing the various components of the US Armed Forces, from nuclear submarines to bombers. It has been codenamed Looking Glass, and continues to serve in this role today. 16 have been built.