The specification for a two-seat, twin-engine jet fighter and a related reconnaissance aircraft was issued by Joseph Stalin on 6 August 1951.

In a break from previous Yakovlev designs, the Yak-120's thin, mid-set wings were swept back at a 45 degree angle with large two-section flaps.

The aircraft's avionics allowed it to navigate and intercept its targets in all weather conditions at altitudes up to its service ceiling.

[2] The aircraft's wings, tail unit, and air intakes were equipped with hot air de-icing, while the foreign object damage protection screens and intake center-bodies of the engines were de-iced electrically, enabling the Yak to remain at colder high altitudes for longer times and to operate in regions with a cold climate.

Despite its complex and heavy avionics, the aircraft had a lightweight airframe for a twin-engined fighter, due to a design that reduced structural weight to the minimum.

In April 1954, a Yak-120 prototype with the RP-6 passed its state acceptance trials, and on 13 May the Council of Ministers approved the production of the modified version with the designation Yak-25M.

In addition to the radar, this version also incorporated several changes – the AM-5A Srs 1 turbojets were replaced with RD-5A (AM-5A) Srs 2 engines with the same rating, the wheelbase was increased by moving the nose gear unit 33 cm forward to improve directional stability during takeoff and landing, and the cannons were fitted with muzzle brakes.

RAF RPF ELINT flights from October 1956 indicated that the Yak-25M had entered service, but that height-finding by ground-based radars was poor and so interception was ineffective above 35,000 feet.

[6] In September 1953, after the first stage of the Yak-120's state acceptance trials concluded, the second Yak-120 prototype was re-engined with the new Mikulin AM-9A turbojet, an improved version of the AM-5.

Due to delays occasioned by the late delivery of the engines and lack of radar, the aircraft was not completed until the end of 1954, and was instead equipped with a standard RP-6.

However, the aircraft's performance was slightly short of the operational requirements, and the failure to complete the Sokol-M, coupled with a need for supersonic interceptors, forced abandonment of the Yak-120M.

In order to make room for two AKAFU automatic tilting camera mounts installed next to each other, the radar was removed and the nose extended slightly.

It completed manufacturer flight tests in October 1953, and conducted state acceptance trials between December 1953 and February 1954.

The Yak-125 outperformed the production Ilyushin Il-28R and Mikoyan-Gurevich MiG-15bisR, but the state commission recommended modifications and directed the construction of a small pre-production batch for service trials due to pilot and reconnaissance systems operator visibility limitations caused by the longer nose in the case of the former and the small ventral window for the latter.

The reconnaissance systems operator was moved to a compartment in the nose, in which oblique and vertical cameras were installed in front of the seat.

By a Council of Ministers directive of 10 March 1955, which was followed on the next day by an order of the Ministry of Aircraft Production, Yakovlev and Factory No.

[10] Yakovlev was tasked by a Council of Ministers Directive of 18 September 1954 and a Ministry of Aircraft Production order issued three days later with developing a fast maritime reconnaissance version of the Yak-25, designated the Yak-25MR.

Yakovlev was tasked with developing a high-speed nuclear bomber using the basic Yak-125 design, the Yak-125B, by a special joint directive of the Central Committee of the Communist Party and the Council of Ministers.

Yak-125B was the first Yakovlev aircraft to include a 360-degree ground mapping and bomb-aiming radar, the RMM-2 Rubidiy, located in a radome directly below the cockpit.

Due to its twin engines and radar intercept operator, pilots gained more confidence on long missions in the remote northern and eastern areas of the Soviet Union.

[15] As it was intended for low-altitude flying, the Yak-25's service ceiling was too low to intercept the American RB-47 Stratojet, which often flew reconnaissance missions over Soviet territory.