After initial testing, the XB-24 was found to be deficient in several areas. One major failure of the prototype was that it failed to meet the top speed requirements specified in the contract. As built, the XB-24 top speed was only 273 mph instead of the specified 311 mph. As a result, the mechanically supercharged Pratt & Whitney R-1830-33s were replaced with the turbo-supercharged R-1830s. Additionally, the tail span was widened by and the pitot-static probes were relocated from the wings to the fuselage. The XB-24 was then re-designated XB-24B—these changes became standard on all B-24s built starting with the B-24C model.