The first Brooklyn line was built in 1895 and terminated at the intersection of Brooklyn Avenue and Evergreen Avenue. In 1902, the route was bought by the Pacific Electric Railway to be standard gauged, but never was. Following the Great Merger of 1911, control of the route returned to the Los Angeles Railway, who extended the route north and east along Evergreen and Wabash Avenues to the city limits and merged the route with the Ascot Line, a previously unconnected route which ran by way of Main Street, 12th Street, Hooper Avenue, a private right-of-way alongside the Santa Monica Air Line, and Ascot Avenue (a section of which, between Adams Boulevard and 41st Street has been filled in).
In 1922, the route was extended northeast along the newly built Harrison Street (later Ramona Boulevard, and later still City Terrace Drive) to Miller Street in East Los Angeles. In 1939, however, the Euclid Shuttle was expanded to a full-length route (E), to serve this area. The B line's terminus was cut back to Evergreen and Wabash. In 1948, the route was removed altogether.