The B991 connects the A96 and A978 together along Bedford Road in Old Aberdeen. The southern end of the route passes a retail park, whilst the northern end passes part of Aberdeen University.
The B991 originally had a slightly different route. The southern end remains the same, but the route originally turned east along Meston Walk, then did a dogleg south into University Road to terminate on the A92 (now A956) King Street. The number seems to have been moved to its current route in the 1950s with the construction of the A978.