Beginning in the 1920s, many streetcar lines in the Bronx, Manhattan, and the rest of the city began to be replaced by buses, particularly after the unification of the city's three primary transit companies in June 1940.
That year, the railroad company began relinquishing its trolley franchises with the city, to be replaced by buses operated by the subsidiary Surface Transportation Corporation.
The 207th Street Crosstown Line was replaced with the then-Bx19 bus service operating between Broadway-207th Street and Southern Boulevard on January 25, 1948, the same date as the motorization of the Bronx and Van Cortlandt Parks Crosstown Line (today's Bx9).
The route would be operated by Surface Transit until 1956, and by the New York City Omnibus Corporation (later under the brand Fifth Avenue Coach Lines) until the company's routes where taken over by the New York City Transit Authority through its subsidiary Manhattan and Bronx Surface Transit Operating Authority (MaBSTOA) in 1962 after a strike.

As the years progressed, service was eventually consolidated from multiple routes, and was given a single label, the Bx12.
The route saw extensions east to City Island, seasonal service to Orchard Beach, and a branch to the Bartow-Pell Mansion in the Pelham Bay Golf Course, along with the introduction of limited-stop service on weekdays.
The Orchard Beach branches were labeled Bx12A and Bx12B until July 1, 1974, when they were merged into the Bx12 designation.

On April 21, 1989, the New York City Transit Authority presented two proposals to rationalize the eastern terminals of service on the Bx12 to the MTA Board for approval.
The first proposal called for the discontinuation of the summer only Bx12 Golf Course Shuttle, which ran between Orchard Beach and Pelham Bay Golf Course between 8 a.m. and 6 p.m., since it was only used by an average of ten daily passengers.
This change took effect in June 1989.
The second proposal up for approval was the extension of Bx12 trips that terminated at the Pelham Bay Park subway station to the new Gun Hill Depot at Bartow Avenue and the New England Thruway when it opened in September 1989.
This change was intended to rationalize service and establish a relief point for bus operators.
While it increased net costs, it increased operating efficiencies.
Service to City Island remained until around 1990, when through service was discontinued and replaced with a dedicated route from City Island to Pelham Bay, the current Bx29.
The resulting service pattern remains in effect today, with some minor adjustments.

In 1988, due to the reconstruction of the University Heights Bridge, the Bx12 began detouring 0.5 miles (0.80 km) to cross into Manhattan via Bailey Avenue and the Broadway Bridge at 225th Street, as opposed to the direct path via 207th Street.
The route was restored in September 1994 as the vehicle weight restriction on the bridge was removed.
In 1989, limited-stop service began on the Bx12, saving up to five minutes per trip.
In January 1995, an additional limited stop was added at Jacobi Hospital, and the hours of limited-stop service were expanded from 6:57 – 8:39 a.m. and 4:55 – 6:41 p.m. to 6:30 – 9:30 a.m. and 4 – 6:30 p.m.. Extending the span of the limited-stop service to operate all day between 7 a.m. and 7 p.m. was considered, with a plan to revisit the idea if the increase in the span of limited-stop service was successful.
The span of the Bx12 was extended to operate all day between 6:30 a.m. and 7 p.m. on September 8, 1997.
Exactly one year later, on September 8, 1998, the Bx12 began stopping in the Bay Plaza Shopping Center.

In 2004, the MTA in conjunction with the NYCDOT and NYSDOT, performed an initial study on bus rapid transit, with 80 corridors studied citywide.
Five routes were prioritized, including the Bx12 route.
On June 29, 2008, the Bx12 Limited was converted into the Bx12 Select Bus Service.
Initially, during summer months Bx12 SBSs alternated between Bay Plaza and Orchard Beach.
In 2009, Bx12 locals were extended to Orchard Beach during summer months, with all SBSs termi
Beginning in the 1920s, many streetcar lines in the Bronx, Manhattan, and the rest of the city began to be replaced by buses, particularly after the unification of the city's three primary transit companies in June 1940. That year, the railroad company began relinquishing its trolley franchises with the city, to be replaced by buses operated by the subsidiary Surface Transportation Corporation. The 207th Street Crosstown Line was replaced with the then-Bx19 bus service operating between Broadway-207th Street and Southern Boulevard on January 25, 1948, the same date as the motorization of the Bronx and Van Cortlandt Parks Crosstown Line (today's Bx9). The route would be operated by Surface Transit until 1956, and by the New York City Omnibus Corporation (later under the brand Fifth Avenue Coach Lines) until the company's routes where taken over by the New York City Transit Authority through its subsidiary Manhattan and Bronx Surface Transit Operating Authority (MaBSTOA) in 1962 after a strike.
As the years progressed, service was eventually consolidated from multiple routes, and was given a single label, the Bx12. The route saw extensions east to City Island, seasonal service to Orchard Beach, and a branch to the Bartow-Pell Mansion in the Pelham Bay Golf Course, along with the introduction of limited-stop service on weekdays. The Orchard Beach branches were labeled Bx12A and Bx12B until July 1, 1974, when they were merged into the Bx12 designation.
On April 21, 1989, the New York City Transit Authority presented two proposals to rationalize the eastern terminals of service on the Bx12 to the MTA Board for approval. The first proposal called for the discontinuation of the summer only Bx12 Golf Course Shuttle, which ran between Orchard Beach and Pelham Bay Golf Course between 8 a.m. and 6 p.m., since it was only used by an average of ten daily passengers. This change took effect in June 1989. The second proposal up for approval was the extension of Bx12 trips that terminated at the Pelham Bay Park subway station to the new Gun Hill Depot at Bartow Avenue and the New England Thruway when it opened in September 1989. This change was intended to rationalize service and establish a relief point for bus operators. While it increased net costs, it increased operating efficiencies. Service to City Island remained until around 1990, when through service was discontinued and replaced with a dedicated route from City Island to Pelham Bay, the current Bx29. The resulting service pattern remains in effect today, with some minor adjustments.
In 1988, due to the reconstruction of the University Heights Bridge, the Bx12 began detouring 0.5 miles (0.80 km) to cross into Manhattan via Bailey Avenue and the Broadway Bridge at 225th Street, as opposed to the direct path via 207th Street. The route was restored in September 1994 as the vehicle weight restriction on the bridge was removed. In 1989, limited-stop service began on the Bx12, saving up to five minutes per trip. In January 1995, an additional limited stop was added at Jacobi Hospital, and the hours of limited-stop service were expanded from 6:57 – 8:39 a.m. and 4:55 – 6:41 p.m. to 6:30 – 9:30 a.m. and 4 – 6:30 p.m.. Extending the span of the limited-stop service to operate all day between 7 a.m. and 7 p.m. was considered, with a plan to revisit the idea if the increase in the span of limited-stop service was successful. The span of the Bx12 was extended to operate all day between 6:30 a.m. and 7 p.m. on September 8, 1997. Exactly one year later, on September 8, 1998, the Bx12 began stopping in the Bay Plaza Shopping Center.
In 2004, the MTA in conjunction with the NYCDOT and NYSDOT, performed an initial study on bus rapid transit, with 80 corridors studied citywide. Five routes were prioritized, including the Bx12 route. On June 29, 2008, the Bx12 Limited was converted into the Bx12 Select Bus Service. Initially, during summer months Bx12 SBSs alternated between Bay Plaza and Orchard Beach. In 2009, Bx12 locals were extended to Orchard Beach during summer months, with all SBSs termi
Bx12 bus