Question:  North American Aviation created the prototype of the B-70 bomber, capable of traveling at what height?

Context paragraphs:
North American XA2J Super Savage: The North American Aviation XA2J "Super Savage" was a prototype carrier-based attack aircraft built in the early 1950s.  It was developed by North American Aviation (NAA) from the smaller AJ Savage.

General Aviation GA-43: The General Aviation GA-43 was an airliner produced in small numbers in the United States in the mid-1930s, also known as the Pilgrim 150, Fairchild 150, and Clark GA-43.  The prototype was developed and built by Fairchild's American Pilgrim division, but the program was taken over by General Aviation when the firm purchased American Pilgrim shortly before the prototype had flown.  Although this first flight took place in 1932, manufacture did not commence until 1934, by which time General Motors had, in turn, gained a controlling interest in North American Aviation and merged it with General Aviation, which they already owned.  The upshot of this was that the GA-43 became the first aircraft produced by North American.  The GA-43 was a conventional low-wing cantilever monoplane of all-metal construction.  The prototype had fixed tailwheel landing gear, but the main units of this were later changed to be made retractable, and three of the four production examples also had retractable mainwheels, the fourth aircraft having twin pontoons instead.  The oval-section fuselage contained a ten-seat passenger cabin, and the cockpit was located atop the fuselage under a separate canopy.

North American T-6 Texan: The North American Aviation T-6 Texan is an American single-engined advanced trainer aircraft used to train pilots of the United States Army Air Forces (USAAF), United States Navy, Royal Air Force, and other air forces of the British Commonwealth during World War II and into the 1970s.  Designed by North American Aviation, the T-6 is known by a variety of designations depending on the model and operating air force.  The United States Army Air Corps (USAAC) and USAAF designated it as the AT-6, the United States Navy the SNJ, and British Commonwealth air forces the Harvard, the name by which it is best known outside of the US.  Starting in 1948, the new United States Air Force (USAF) designated it the T-6, with the USN following in 1962.  It remains a popular warbird aircraft used for airshow demonstrations and static displays.  It has also been used many times to simulate various Japanese aircraft, including the Mitsubishi A6M Zero in movies depicting World War II in the Pacific.

North American NA-16: The North American Aviation NA-16 was the first trainer aircraft built by North American Aviation, and was the beginning of a line of closely related North American trainer aircraft that would eventually number more than 17,000 examples.

North American P-51 Mustang: The North American Aviation P-51 Mustang is an American long-range, single-seat fighter and fighter-bomber used during World War II and the Korean War, among other conflicts.  The Mustang was designed in 1940 by North American Aviation (NAA) in response to a requirement of the British Purchasing Commission.  The Purchasing Commission approached North American Aviation to build Curtiss P-40 fighters under license for the Royal Air Force (RAF).  Rather than build an old design from another company, North American Aviation proposed the design and production of a more modern fighter.  The prototype NA-73X airframe was rolled out on 9 September 1940, 102 days after the contract was signed, and first flew on 26 October.

North American AJ Savage: The North American AJ Savage (later A-2 Savage) was a carrier-based medium bomber built for the United States Navy by North American Aviation.  The aircraft was designed shortly after World War II to carry atomic bombs and this meant that the bomber was the heaviest aircraft thus far designed to operate from an aircraft carrier.  It was powered by two piston engines and a turbojet buried in the rear fuselage.  The AJ-1 first became operational in 1950 and several were based in South Korea during 1953 as a deterrent against the Communists.  Of the 140 built, plus three prototypes, 30 were reconnaissance aircraft.  Inflight-refueling equipment was deployed on the Savage in the mid-1950s.  The bomber was replaced by the Douglas A3D Skywarrior beginning in 1957.

North American FJ-1 Fury: The North American FJ-1 Fury was the first operational jet aircraft in United States Navy service, and was developed by North American Aviation as the NA-135.  The FJ-1 was an early transitional jet of limited success which carried over similar tail surfaces, wing, and canopy derived from the piston-engined P-51D Mustang.  The evolution of the design to incorporate swept wings would become the basis for the land-based XP-86 prototype - itself originally designed with a very similar straight-wing planform to the FJ-1 airframe - of the United States Air Force's enormously influential F-86 Sabre, which itself formed the basis for the Navy's carrier-based North American FJ-2/-3 Fury.

North American Aviation: North American Aviation (NAA) was a major American aerospace manufacturer, responsible for a number of historic aircraft, including the T-6 Texan trainer, the P-51 Mustang fighter, the B-25 Mitchell bomber, the F-86 Sabre jet fighter, the X-15 rocket plane, and the XB-70, as well as Apollo Command and Service Module, the second stage of the Saturn V rocket, the Space Shuttle orbiter and the B-1 Lancer.  Through a series of mergers and sales, North American Aviation became part of North American Rockwell, which later became Rockwell International and is now part of Boeing.

North American XB-21: The North American XB-21, also known by the manufacturer's model designation NA-21, and sometimes referred to by the name "Dragon", was a prototype bomber aircraft developed by North American Aviation in the late 1930s, for evaluation by the United States Army Air Corps.  Evaluated against the Douglas B-18 Bolo, it was found to be considerably more expensive than the rival aircraft, and despite the ordering of a small number of evaluation aircraft, only the prototype was ever built.

North American XB-70 Valkyrie: The North American Aviation XB-70 Valkyrie was the prototype version of the planned B-70 nuclear-armed, deep-penetration strategic bomber for the United States Air Force Strategic Air Command.  Designed in the late 1950s by North American Aviation, the six-engined Valkyrie was capable of cruising for thousands of miles at Mach 3+ while flying at 70,000 ft .

Supporting paragraphs: North American Aviation North American XB-70 Valkyrie
Answer: 70,000 ft