On August 18, 2016, William Byron and Hendrick Motorsports announced a multi-year driver development agreement, with Byron running full-time in the Xfinity Series driving the No. 9 Liberty University Chevrolet Camaro for JR Motorsports in 2017. It was the first time since 2014 that the No. 9 was driven by a rookie driver. At the 2017 Ford Eco Boost 300, as William Byron and Elliott Sadler were battling late, Byron took advantage of Sadler's mistake of trying to pass Ryan Preece when he was too far back, slowing Sadler and allowing Byron to pass both drivers. When Sadler tried to follow, he made contact with Preece, sending Preece sideways and Sadler into the wall. After that Byron pulled away and finished 3rd, ultimately winning the championship and Rookie of the Year honors while winning four races at Iowa, Daytona, Indianapolis and Phoenix.