Was there ever a YF-24? The US Air Force says no. "Our historians said there is no record of there ever having been a YF-24," says Lt Col Max Despain, an Air Force spokeswoman at the Pentagon. "Perhaps it's being mistaken for an X-24 which wasn't a fighter?"
That said, this old bio for a former test pilot, Colonel Joseph A. Lanni lists such an aircraft--which is curious. So there remains a small possibility a YF-24 might have been squirreled away somewhere out in the desert in Nevada. I say that because Lanni, according to his bio, commanded a classified flight test unit between July 1995 and June 1997.
While the entry in the bio might be a typo, it was certainly not the X-24 that Lanni flew. The Martin Marietta X-24A flew during 1963 to 1975, which was well before Lanni became an Air Force pilot. It's also not likely that Lanni flew the Northrop YF-23 either since he was assigned to Eglin AFB in Florida during the time those jets were flying.
So, what exactly the YF-24 is or was is still kind of a mystery. Your guess is as good as mine.
But I did find this intriguing Boeing Multirole Fighter concept design called the Model-24F, which seems to have preceded the Joint Strike Fighter program. Certainly the design shares traits with earlier Boeing concepts from the Advanced Tactical Fighter program and with later efforts such as the ill-fated X-32 and more recent F/A-XX concepts the company has been showing-off.
From the specifications, it was clear that at the time the Model-24F was being examined, the USAF was looking for a much lighter and much faster aircraft than the current Lockheed Martin F-35. It's apparent from these diagrams that the Boeing Model-24F design was to be a much more slender aircraft than what eventually became the JSF. It also looks like it was designed to be far more agile--it has provisions for two-dimensional thrust vectoring and some other interesting features. The design matches the Raptor's top speed of about Mach 2.2 though it doesn't cruise supersonically like the F-22.