The B370 originally ran from the Surrey side of Hampton Court Bridge (opposite Hampton Court Palace) to Weybridge.
Starting on the A309 in East Moseley the road ran west following the River Thames not particularly closely. After passing through the water works it reached Walton-on-Thames and the A244. There was a short multiplex west along Bridge Street, that road's original route, before the B370 reappeared to head south along Oatlands Drive. It wasn't long before Weybridge was met, where the road ended on the A317 to the east of the town centre.
The section from Walton-on-Thames to Weybridge was upgraded to A3050 in the 1920s, but the B370 number survived on the remainder of the route for many years after this until it was also upgraded to an extension of the A3050.