The club's first success in this early period came in 1928, when RSK Borac won a tournament played in Sarajevo. Before World War II, the major club in Banja Luka was ŠK Krajišnik, however, after 1945, it was disbanded, and Borac replaced Krajišnik as city´s main club. Borac played conference leagues in order to qualify for the reestablished Yugoslav First League where defeated Sloboda Novi Grad 14–3 on aggregate, FK Kozara from Banja Luka with 8–4 on aggregate and Borac Kozarska Dubica 7–0 on aggregate and became Banja Luka district champions. In 1945, RSK Borac was renamed in FK Borac Banja Luka and played for two years in the Yugoslav Third League, before it was promoted to the Yugoslav Second League in 1953.