The development of the MiG-25 began in the 1950s, paralleling American efforts to develop a Mach 3 bomber and interceptor aircraft, including the (ultimately abortive) XB-70 Valkyrie, the XF-103 Thunderwarrior, the Lockheed YF-12, and the XF-108 Rapier. With the first Mach 2 aircraft beginning to enter service, Mach 3 seemed like the next logical step. A variety of roles were considered, including cruise missile carriers, and even a small five- to seven-passenger supersonic transport, but the main impetus was a new high-altitude reconnaissance aircraft and heavy interceptor. If a Mach 3 bomber were to enter American service, it would be nearly invulnerable to Soviet air defense.

The Mikoyan-Gurevich OKB accepted the assignment effective 10 March 1961. Although the XB-70 Valkyrie was cancelled before the new aircraft, which carried the bureau designation Ye-155 (Russian: Е-155), reached the prototype stage, it seemed that the Ye-155 would still be a useful addition to the PVO interceptor force against reconnaissance targets like the SR-71 Blackbird. It was widely believed that the MiG-25 was developed in response to the XB-70. However, A. Belyankov, head of the MiG design bureau confirmed that this was not in fact the case.

The first prototype, which was actually the reconnaissance variant designated Ye-155-R1 by the bureau, made its first flight on 6 March 1964. The prototype interceptor, Ye-155-P1, took to the air on 9 September 1964. Development, which represented a major step forward in Soviet aerodynamics engineering and metallurgy, took several more years to complete. In the meantime, several prototypes, under the cover designation Ye-266 (Russian: Е-266), made a series of record-setting flights in 1965, 1966, and 1967.

As for the XB-70,
In December 1959, the entire program was cut back to a single prototype. This wasn’t the last word on the matter, though, since big weapons procurement efforts acquire a momentum of their own, and by mid-1960 funding for the B-70 program had been restored to a level adequate for as many as a dozen of the bombers.

On 1 March 1961, US President John F. Kennedy announced that the B-70 program was to be scaled back once more. Three aircraft would be completed, including two “XB-70” flight test prototypes and one “YB-70” operational prototype.

Therefore, the specification and request for proposal (or the Soviet equivilant) was issued while the XB-70 was still a viable production program!

And yes, the SR-71 (A-12 originally) was the result of a 1957 CIA request, with Lockheed recieving the order for the aerodynamic & prototype work in 1959 and the order for the first 12 on January 30, 1960… but as this project was a very secret one.

You really have to give me references proving that the USSR had learned of it before March 10, 1961 to convince me that the E-155 was originally ordered to counter the Blackbird!

Yes, there were more reasons than just the Valkyrie for the continued development of the E-155/Mig-25, but it was a major reason the project was started!

Was the MiG 25 also developed to counter our own (British) Avro 770 supersonic bomber?

As others have pointed out the F-15E and Tornado GR4 both need replacing. In Britain there’s been some talk of UAVs and increased orders for Typhoons and Lightning IIs but buying some FB-22s along with the US might not be a bad idea.

She operated until 1967 when following a trivial fire after routine shipyard work, the government ordered her decommissioned. Many of her crew were transferred to HMS Hermes, then recommissioning.

This was as result of 2 factors:
1. The FAA had started acquiring Phantoms (F-4K/FG.mk.1), and Vicky was determined to be too small to be modified to operate them.

2. This was just after the cancellation of the CVA-01 project, and the MOD had decided that long-range land-based strike aircraft (F-111K) would make carrier-based Tac-air unnecessary. The decision had been made to focus primarily on keeping the Ark Royal & Eagle in service, and the Hermes had just completed a refit that allowed her to carry the Buccaneer, so there was little loss of capability by “replacing” Vicky with Hermes (same types of aircraft, but fewer).

Actually, Victorious could carry Buccaneer S2s just like Hermes. And like Eagle ‘Phantomisation’ would have (only?) required the following additions/alterations to her flight deck equipment, new Jet Blast Deflectors, Van Zelm bridle cathers, 4 direct acting arrestor wires and possible an AN/SPN 35 CCA radar. Removal of her remaining gun armament and replacing it with some additional extentions to her flight deck aft of the Island and possibly on the port-side aft of the overhang of the anged deck might have also increased deck parking space.
Also there was serious talk of operating Phantoms from Hermes and that ship did cross-operate with US Navy F4s.

I was wondering. If the Argentines ever did build or purchase a carrier and high-performance warplanes to operate from it, what, if any impact would this have on the Royal Navy’s CVF program? Would it make it more likely that the’d press ahead in order to counter Argentina (who, lets face it have never given up hope of annexing the Falklands)?

Its called P=1154 but due to interservice bickering and problems with something called Plenum burning in the engines which lead to shortcomings in performance as well as cost and deadline over-runs it was cancelled in the mid 1960s. Funnily enough, had it been built it would have been called Harrier.