Lots of high speed delta's were on the drawing board from just after WWII, both British and American, but as the American effort never got further that a chipboard mock up, lets not go there eh.

Boeing and its engine makers General Electric could not resolve the engine air intake issues that plauged supersonic engines. British aerospace together with their French partners however did, and the rest is history.

As to the SR-71. Yes a nice plane, which would have been a lot nicer if it did not leak fuel all over the floor before take off and not have an enormouse radar signature that the Russians could see coming just after take off.

Also the fact that at full speed it could take up to 80 miles to do a 180 degree turn, really meant it was nothing more than a powered cruise missile useful for reconosance missions only.