In 1953 the Soviet authorities issued a requirement for a Mach 2 clear-weather interceptor with limited ground-attack capability. At this time the USSR’s Central Aerodynamics and Hydrodynamics Institute had arrived at two basic configurations for aircraft of the required performance level. Both were based on a cylindrical fuselage with a swept all-moving tailplane and a wing in the low mid-set position, but the difference came in the wing itself. One was a conventional type with a leading-edge sweep of between 580 and 620, and other a delta with 570 or 580 leading-edge sweep. The MiG bureau produced prototypes in both configurations. The Ye-50 and Ye-4.
An RD-9Ye turbojet was used in the Ye-4 with a delta wing, which first flew in December 1955. The Ye-4 was the true progenitor of the MiG-21, becoming the Ye-4/2 with airflow fences.