In regards to the differences between the XB-70 (A/V-1) and XB-70 (AV-2), the original A/V-1's hydraulic lines were fixed and not flexible, resulting in a number of leaks. The wing featured a mostly neutral wing (some anhedral was present at the wing-body junction), and the plane featured a manual only inlet control system. The metal skin featured a number of defects resulting in it being limited to a lower speed. The radome under the nose was also painted white.
-2 featured flexible hydraulic-lines, it's wings featured 5-degrees dihedral, the folding-tips folded an extra five degrees in the mid-position and the maximum position. The inlet control system was automatic, with manual override. The metallic skin was developed with experience learned from AV
-1. As a result, far less if any problems occured with AV
-1. The radome under the nose of A/V-2 was black instead of white, being one of the easiest ways to identify the two.
Their tail numbers are also different as well. AV
-1 was designated 62-0001, which was written out as 20001 on the tail. AV
-2 was designated 62-0207, which was writen out as 20207.
A commonly mistaken fact is that the XB-70 is capable of only Mach 3.0. In a respected book about the XB-70, plus an article written on a NASA website regarding the XB-70's inlets, indicate a much higher speed.
The book states that the X-279E (YJ-93) was rated for Mach 4 performance. The competitor as well, the Boeing 804-4 was designed additionally for Mach 4.0 performance. Considering that the XB-70 won, I doubt even with extra bomb-capacity and a better L/D ratio that it would have won with only Mach 3-speed. The high-speed capability was highly important!
The NASA site discussed an inlet design they tested on the XB-70, that would have produced a ninety-percent effciency at Mach 3.8. NASA would have no reason to overrate the speed of the XB-70 (especially since normally 3.0 is the listed figure) to such an extent, but they might have had a reason to under-rate it. After all it would be pretty amazing to know that even in the late fifties we had jet-enignes that could cruise (well continuously dash) at mach 4.
Considering the two of them do appear to be worthy of trust. It would be logical to conclude that the XB-70's cruise-speed would be either Mach 3.8 to Mach 4. Considering that the plane's likely cruise speed is Mach 4, or close too, I'd assume it would cruise at a considerably higher altitude than 75,000 feet.
Regarding the jet engine issue, the X-279E/J-93, and it's competitor, the J-91, which was known as the JT
-9 (dunno if it's related significantly to the JT
-9D) was also capable of similar speed. It featured a lower pressure ratio (7:1) since it didn't feature the advanced air-cooling system the J-93 featured. To compensate, the engine was bigger in diameter than the J-93. Since the J-93 ultimately was considerably more advanced, it won the competition. However before this happened, the Navy was in need of a high-mach engine to power an interceptor design of theirs, which I've been told was either an F-8U
-III variant, or an advanced version of the A-3J (Later A-5 Vigilante), Pratt 'n Whitney developed an 80% scale version of the J-91 which was designated J-58. (To provide the smaller engine with rivalable power, they designed it with a variable inlet-guide vane to allow the blades to operate at a higher alpha and thus produce a higher PR, and once the compressor inlet temp rose past a given point, the guide-would shift to lower the alpha, reducing the pressure ratio to a lowever value. Allegedly it's thrust was around 26,500 lbf dry, and 45,000 lbf wet.)
. During the Valkyrie's development, they did sometimes consider using the J-58 as a back-up choice should the J-93 for one reason or another fail. For this reason, it is logical to assume the J-58 had around the same thrust and same top speed as the J-93 (M=4.0).
This fact also makes it fairly obvious that the A-12/YF-12A/SR-71 are obviously faster than Mach 3.2, or Mach 4.0, since the engines were heavily modified from their original capabilities.