As an interim step to allow evaluation of turbine power, the first prototype was given a mixed powerplant comprising two 1025kW Allison V-1710-133 piston engines to drive the propellers, plus two 726kg thrust Westinghouse 19XB-2A turbojets mounted in underwing nacelles. Redesignated XB-42A, this aircraft was used for performance testing over several months before being retired at the end of June 1949.