The redesigned aircraft, dubbed the F-105B, proved able to achieve speeds of Mach 2.15. Also included were improvements to its electronics including the MA-8 fire control system, a K19 gun sight, and an AN/APG-31 ranging radar. With the alterations complete, the YF-105B first took to the sky on May 26, 1956. The largest single-engine fighter built for the US Air Force, the production model F-105B possessed an internal bomb bay and five external weapons pylons. The US Air Force planned to purchase 1,500 F-105s, however this order was reduced to 833 by Secretary of Defense Robert McNamara.