The streetcar line, the first street railway in the Bronx, was known as the Harlem Bridge, Morrisania, and Fordham Railroad.

In 2013, the Bx55 was converted into the Bx15 Limited, extending the route south to Manhattan, but eliminating service north of Fordham Plaza.

[7] The Bx15 connects the two largest shopping districts in the Bronx: Fordham Road and the Hub.

[8] The M125 bus route begins at the Hub, traveling south on Willis Avenue.

[13] The M125 fleet is from Mother Clara Hale Depot and uses Nova articulates and New Flyer XD60s.

[15] In 1863, the first street railway was constructed in the Bronx, known as the Harlem Bridge, Morrisania, and Fordham Railroad.

On August 23 of that year, it was granted several streetcar franchises in the Bronx by the Board of Aldermen, as part of an expanded "Huckleberry System".

[16] In February 1904, the Union Railroad would apply for extensions of several streetcar lines into Manhattan.

[19] Ten years later on July 2, 1914, the company received a permit to extend Willis Avenue service across the bridge, then west along 125th Street to Fort Lee Ferry, which connected Manhattan with Fort Lee, New Jersey.

The new traffic pattern was instituted on the morning of August 5, 1941, and bus service along the former streetcar route began that afternoon.

In 1995, New York City Transit was in the process of building a weather-protected intermodal terminal at Third Avenue–149th Street.

Supplemental Bx15 service was provided between Fordham Road and 149th Street to maintain the Bx55's frequency.

In addition, an unnecessary turn at 147th Street was eliminated; it had existed because a support pillar of the Third AvenueI Elevated, which had been demolished for some time, had prevented northbound buses from turning directly from Willis Avenue to Third Avenue.

[45] In December 1999, the MTA announced a plan to make a slight route change in southbound Bx15 service between East 138th Street and the Third Avenue Bridge.

[47] In February 2004, selected weekday and all weekend Bx55 trips terminated at Fordham Plaza.

[49] As a result, the Bx15 became an articulated bus route in order to hold passengers dispatched from the Bx55.

[8][51] On the same day, the Bx55 was discontinued and, on July 1, it was replaced by new Bx15 limited-stop service, which operated at all times except late nights and weekends.

Service north of Fordham Plaza along Webster Avenue was replaced by the Bx41 SBS.

[56] As part of the MTA's 2017 Fast Forward Plan to speed up mass transit service, a draft plan for a reorganization of Bronx bus routes was proposed in draft format in June 2019, with a final version published in October 2019.

The plan included splitting the section of the Bx15 south of the Hub into a new bus route labeled the M125,[57] which was proposed to be implemented by mid-2020.