The B731 is a short link road which runs to the west of the small town of Thornhill in Nithsdale. It starts at a T junction on the A76 about half a mile southeast of Thornhill, and heads north west, initially following the Cample Water downstream through trees. Although there is a centre line, the road is narrow and windy, meaning it is not the western bypass for Thornhill that it looks on a map! A left turn at a crossroads crosses the river, while right leads into the town. After this, the route starts to turn further north, with the fields to the left now stretching down to the Nith, while Thornhill sits up on the increasingly steep hill to the right. A long straight then leads up to a junction on a bend of the A702 (former B730) just east of the Nith Bridge. A minor road leads right into the town centre, while a short distance to the north, the A702 also turns into the town, with the B732 continuing the pseudo-bypass back to the A76.
The 1922 MOT Road List defines this route as: Nith Bridge - Junction with A76 South of Thornhill
