The alignment of Route 99 was originally designated as part of New England Interstate Route 10 in the 1920s. The Silas Deane Highway was built in 1930 and New England Route 10 was shifted slightly west to use the new highway.
Route 99's alignment was initially designated as part of New England Interstate Route 10 in the 1920s. In 1930, the Silas Deane Highway was constructed, and New England Route 10 was shifted westward to utilize this new highway.