What are the differences between the MiG 31 and MiG 25 (if any)?
The MiG-31 is an all new design with higher strength for low-level flight and new engines. The reason it looks just like the MiG-25 has to do with the structure of the Soviet aviation industry. The basic shape is first defined by the Central Aerodynamic and Hydrodynamic Institute (TsAGI is the abbreviation of its Russian name) and then given to the design bureaus which do the mechanical design. Production is then relegated to dedicated manufacturers which build the designs of different bureaus in parallel.

Both aircraft were designed as interceptors. The main difference is the advanced radar, digital data links and the more powerful engines of the MiG-31 which made a longer fuselage necessary. The MiG-25 radar, a development from the 1960s, could only detect high-flying targets, while the Zaslon radar of the MiG-31 can also detect and track low flying aircraft (look-down/shoot-down capability); a necessity after NATO changed its strategy to nap-of-the-earth flying to avoid detection.

If you need to tell both apart on pictures, look for these features:


While the main landing gear of the MiG-25 has a large, single wheel, the main gear of the MiG-31 uses two smaller wheels in a staggered arrangement.




MiG-31 main landing gear (picture by Burkhard Domke source)


While most MiG-25 were single seaters, the MiG-31 has two occupants, a pilot and a weapon systems officer behind him. Two-seater versions of the MiG-25 can be easily distinguished because the two cockpits are separate units, the front cockpit replacing the radar.




MiG-31 on the ground with canopies open (picture source)


The wing root of the MiG-31 has a small strake added while the wing of the MiG-25 is a pure trapeze with a straight leading edge. The picture below shows the MiG-25 in black lines and the MiG-31 overlaid in red.