Notice the absence of the chines that on the SR71 came all the way to the nose cone? My understanding is that adding the chines to the SR71 removed the need for a ventral fin. Lockheed convinced the USAF that an aircraft based on the A-12 would provide a less costly alternative to the recently canceled North American Aviation XF-108, since much of the design and development work on the YF-12 had already been done and paid for.