The R-25 is a two-spool axial-flow turbojet with a new compressor with increased overall pressure ratio and airflow, variable two-stage afterburner, and greater use of titanium.

An unusual addition to the R-25 was an emergency mode thrust boost which increased the compressor speed to 106% and also increased the afterburner fuel flow with the addition of a second afterburner fuel pump.

Thrust was increased to 96.5 kilonewtons (21,700 lbf) below an altitude of 2,000 metres (6,600 ft).

Use of this WEP required the engine to be removed on landing for inspection.

The engine was also built under license by HAL in India for its fleet of MiG-21bis.