It's interesting to note though that although Ben Rich explains that they decided they wouldn't bid, Lockheed WERE one of the airframe contractors in Phase 2 and it's difficult to imagine that the Skunk Works weren't a part of that effort especially given the work they did towards the scramjet powered X-24c in the seventies.

Also, they went on to the X-33 in the mid 90's so clearly felt confident in a suitable "something stronger" thermal protection system by then