Republic Airways operated its first flights out of Chicago O'Hare Airport as American Eagle on Thursday.
The regional carrier is flying new 76-seat Embraer E-175 aircraft for American Airlines as part of its 12-year partnership with American.
The new aircraft will initially fly from Chicago to Albuquerque, New Orleans and Pittsburgh. American plans to add more E-175s through Republic with 47 aircraft in service by the beginning of 2015.
"For the first time in American Airlines history we are offering large regional jet flying as an option," said American's vice president of network planning Chuck Schubert.
The E-175 aircraft will have 12 first class seats, 20 main cabin extra seats and 44 main cabin seats, the carrier said.
Republic has previously flown regional flights for American out of Chicago under the American Connection brand.
During its bankruptcy process, American has renegotiated its union contracts to enable it to hire more third-party regional carriers to fly short-haul flights under the American Eagle brand. The Fort Worth-based carrier plans to transition out of its older ATRs and E-135 aircraft that it currently uses for regional flying by the end of 2013.