Disappointed at the XB-24 prototype's relatively slow maximum speed of 273 mph instead of the 311 mph originally estimated, on July 26, 1940, the Army recommended that some changes be made to the aircraft to improve its performance, especially at high altitude.
First and perhaps most important of these changes was the introduction of engine turbosuperchargers to give the aircraft a better high-altitude performance. Accordingly, the XB-24 was re-engined with 1200 hp Pratt & Whitney R-1830-41 (S4C4-G) radials which were equipped with General Electric B-2 turbosuperchargers which replaced the mechanical two-speed superchargers of the earlier engines. The turbosuperchargers were mounted on the lower surface of each engine nacelle. The oil coolers were relocated to the flanks of the front cowlings and the air intakes for the turbosuperchargers were placed on the sides of the engines, which gave the nacelles a characteristic elliptical cross-section, which remained with the Liberator all throughout its production career.
The Army also directed that Consolidated eliminate the "wet" wing and install self-sealing fuel tanks in the wing. The engine controls were modified so that they permitted at least 60 percent engine power even if the controls were shot away. Electrical engine primers were added, and the engine nacelles were redesigned. The tail span was increased by two feet.
With all these changes, the XB-24 was redesignated XB-24B. The first flight of the XB-24B took place on February 1, 1941. The performance was markedly improved. With turbosuperchargers fitted, the original takeoff power of 1200 hp could be maintained to well above 20,000 feet, resulting in an increase in maximum speed to 310 mph. Turbosuperchargers remained a feature of all later production Liberators.
Problems with the R-1830-41 engines led the Army to replace them with the more reliable 1200 hp Pratt & Whitney R-1830-43 engines.