This Day in McDonnell Voodoo History, February 25th:

In 1955, the IF-101A interceptor program was officially endorsed by Headquarters USAF.  This would lead to the F-101B version that served into the mid-1980s.

Photo: The IF-101A was one of two different Voodoo interceptor proposals considered for production as a backstop to the continually delayed F-102 program.  While the proposed IF-101B had systems more along the lines of the "ultimate" F-102B interceptor (to become the F-106), The IF-101A was a minimum-change version of the basic F-101A that would borrow the fire control system and armament of the "interim" F-102A  to provide a high-performance long-range interceptor with a relative minimum of technical risk.  Within a short period of time, the IF-101A concept would undergo rapid evolution.  Here, the J57-P-13 engines and "pitch-up fillets" that first appeared on Block 15 F-101A aircraft are visible.

Photo Credit: McDonnell Douglas via Bert Kinsey / Detail & Scale