Schwinn first introduced the balloon tire (2.125" wide) to the US market in 1933 and most of the early converted mountain bikes were based on the pre-war Excelsior-type frames.  The first hand built bikes used the geometry from the early Schwinns as well.  Despite all this "history", Schwinn did not pick up on the mountain bike trend and many feel it was a major contributing factor to their bankruptcy.  If you check out the specification sheets below you will notice that the early Schwinn bikes were Varsity type "gas-pipe" frames with limited gearing, one-piece cranks, super high-rise handlebars and caliper brakes.  These were not specs that would excite a mountain biker and made the bikes best suited to town or beach riding.  Starting with 1985 I am just listing from the Sierra model and up.

The 1980 offerings consisted of a 5 speed cruiser style bike:

In 1981, the King Sting was added which was more of a 5 speed BMX cruiser than true mountain bike.  The 5 speed Cruiser continues.

For 1982, the King Sting is available in a 10 speed model but there is still no true mountain bike available from Schwinn.  The Sidewinder 5 and 10 models are added as a lower priced alternative to the King Sting.  The 5 speed Cruiser soldiers on.

June 1984 High Sierra Ad:

1985 Schwinn Mountain Bike catalog:

Bike reviews from the June 1985 Mountain Biking for the Adventure magazine including the Schwinn Cimarron.

June 1988 Cimarron ad:

1990 Schwinn mountain bike catalog:

Click the picture below for a 4 page PDF file describing the 1998 Schwinn Project Underground bike:

Some of the scans above were copied from Tom Findley's web site.