in spring of 1951. The first M-4 (Bison-A) prototype flew on 20 January 1953, and was handed over to state acceptance trials in March 1954, with production beginning later that year. It entered service in 1955, with 34 being built including two prototypes. The M-4 was made mostly of aircraft aluminum alloys with some steel and magnesium components. It had wings swept at 35-degrees and powered initially by four Mikulin AM-3A engines with a maximum thrust of 85.8 kN (8,750 kgp; 19,290 lbf), but later upgraded to RD-3M-500 turbojets with a maximum thrust of 93.2 kN (9,500 kgp; 20,940 lbf). There were 18 bladder fuel tanks in the fuselage and wings, providing a total fuel capacity of 123,600 liters (32,610 US gallons); this gave the aircraft a range of , although this fell short of the range initially specified. It had a payload of 24 tonnes (26.4 tons) in various configurations. Defensive armament consisted of six AM-23 23 mm cannons with a rate of fire of 1,250 rpm each in a manned twin tail turret with 400 rounds per gun and two twin remote controlled turrets in the top and bottom fuselage with 300 rounds per gun each. The aircraft had a crew of eight: a navigator/bombardier in the nose; pilot and copilot in the cockpit; radar operator/navigator, flight engineer/gunner, radio operator/gunner, and dorsal turret gunner in a compartment behind the cockpit; and a tail gunner. 3M While the M-4 had less range than the Tupolev Tu-95, it had greater speed and payload, sufficient advantages to continue improving on the design. In 1954, approval was granted for a redesign of the M-4, which flew on 27 March 1956 and began state trials in early 1958. The 3M "Bison-B" was powered by four Dobrynin RD-7 turbojets, which had the same thrust as the RD-3M but were 25% more fuel efficient; a nose inflight refueling probe was also added to further increase range. The center fuselage was considerably redesigned to reduce weight and improve aerodynamics, and the wings were updated with a wider span and area. 74 Bison-Bs would be built. Tanker From the outset when the M-4's range shortfall became apparent, Myasishchev began investigating inflight refueling (IFR). In 1955, the second production aircraft was modified to a hose-&-drogue tanker configuration and the first production aircraft was fitted with an IFR probe above the nose, with two more converted for trials the