In 1937 the Navy placed an order for forty additional SBU’s.  These were designated SBU-2 and featured the improved Pratt & Whitney R-1535-98 Wasp engine.  The SBU’s gradually replaced the SU series in service.  However, the airplane never achieved the widespread usage of its predecessors.  One reason was that the Navy began the transition to monoplanes in lieu of biplanes, as the SBU was becoming operational.  The increased threat of war in Europe hastened the introduction of monoplanes with better performance and improved characteristics.  As a result, most of the SBU’s served in stateside training roles, and most were phased out of service in the early 1940’s.