In the 1920s, Route 35 was part of New England route NE-3, a progenitor to today's US 6.
The modern Route 35, commissioned in 1932, has changed slightly in the intervening years. An old rotary at US 7 no longer exists; and the spot where it crosses the New York state line has changed twice.
In 1932, Route 35 followed West Lane all the way from Main Street in Ridgefield to the New York state line. (New York would not designate a continuation of Route 35 until about 1942). A year or two later, Route 35 was moved to South Salem Road, and the remainder portion of West Lane became Route 35A. In 1963, Route 35A became SR 835, which exists today. New York never designated a Route 35A on its side.
In 1940, Route 35 was moved slightly southward near the state line; Old South Salem Road is the older alignment. The new road had been designated SR 434 in 1939.
An old Post Road
The late 1930s Route 35 (West Lane, South Salem Road and Old South Salem Road) was known as Bedford Road in the 1700s; if you followed that road into New York State (where it changes name to Old Post Road), you would move west then south to Bedford, N. Y. However, Route 35 takes a more westerly route to Yorktown.
This Post Road continued from New York State into Ridgefield along today's Route 35, then continued north into Danbury, where an old "Boston Post Road" east of US 7 marks part of the old route. This branch is not one of the three primary routes of the Boston Post Road from New York City to Boston. When the Danbury and Ridgefield Turnpike was laid out in 1801, postal traffic moved to that route.
Proposed freeway for Route 35 (or 34, or 110)
Grander plans were in the wings for Route 35 a few decades ago. A 1956 New York plan called for a Route 35 freeway from Peekskill to Ridgefield, following CT 102 to meet the proposed US 7 freeway.
In 1966, the Tri-State Transportation Commission proposed an expansion of the freeway to New Haven via Monroe, Derby, and the Route 34 alignment. So would it be called 35, or 34? The TSTC instead called it the "Conn. 110 Expressway," citing its benefits as:
East-west route through the former Fairfield County. Fills 18-mile gap in regional highway grid. Connects rapidly developing suburbs. Extends stub-end expressway.
A few comments: "former Fairfield County" alludes to Connecticut's dissolution of county governments in 1960 (no, Westport did not slide into the sea). The "stub-end expressway" is Route 34 in New Haven. Route 110 does have an east-west portion in the corridor between Monroe and Derby, and is closest to the proposed alignment in that area. And finally, the TSTC was at the time quite interested in completing highway grids, filling gaps, and other geometric temptations. This made for some interesting ideas that had little chance of realization. (Also, note that the earlier TSTC has no relation to the modern-day Tri-State Transportation Campaign (http://www.tstc.org), which espouses a more environmentally friendly, less highway-centric approach to transportation planning.)
In 1969, the new Conn. 110 Expressway proposal surfaced in a dispute about a planned interchange at Florida Hill Road and the proposed US 7 freeway in Ridgefield. Residents nearly unanimously opposed the interchange at a town meeting, saying that 1) there were already two nearby interchanges proposed, at Route 102 and old US 7; and 2) this interchange looked like it was really in preparation for the Conn. 110 Expressway.
A DOT engineer replied that the Route 110 proposal was only "a line on a map", and that the state had no inclination to proceed with it – especially if residents did not want it. Furthermore, the Florida Hill Road interchange would be too small for crossing freeways; and the study line had Route 110 crossing US 7 to the south, at Route 102.
In 1975, the Regional Plan Association urged Connecticut not to proceed with its part of the plan. I haven't seen mention of reviving it since then.
Other extension plans
In June 1973, DOT officials told Ridgefield officials that "there no longer exists any plan to extend Route 35 through Ridgefield or to expand Route 116." I don't have details of these plans, though.