The first bus traveled on 8th Avenue, replacing the electric trolley on November 12, 1935.

November 13, 1935	November 7, 1935

When discussions of 8th Avenue changing to a one-way venue were occurring, the bus companies were the biggest opponents to the change.
(See the one-way conversion page for those discussions...)

These two websites are definitive sources for an online history of New York City buses:
http://www.mta.nyc.ny.us/nyct/facts/ffhist.htm
http://www.nycsubway.org/bus/busroutes.html

The Metropolitan Transit Authority website