I've seen it listed as 1/175 - I'm not sure which variant it represents though.
Check these out:
http://www.oldmodelkits.com/index.php?detail=5072&manu=Revell&searchtext=b-52
http://www.oldmodelkits.com/index.php?detail=9429&manu=Revell&searchtext=b-52
I don't remember the X-15 in the kit, just the basic B-52.
I do remember seeing a X-15 at the 1964 World's fair in N.Y.C.
They also had on display one of the Firebird turbine cars.