To meet a Soviet Naval Air Force specification in the late fifties for an antisubmarine helicopter for ship or shore-based use, the Kamov bureau developed a helicopter powered by twin turbines installed side-by-side above the cabin, which drove two three-bladed coaxial, contra-rotating rotors as on their other aircraft. It was first seen at the Tushino air display in July 1961 and was assigned the NATO reporting name Harp. The prototype may have been designated Ka-20.

The Harp was characterized by a large radome under the nose and a fairing beneath the tail boom. The armament consisted of two fixed machine guns in the nose and two small air-to-surface missiles (probably dummies) at the sides of the fuselage. The production version, designated Ka-25, differed from the prototype only in minor details, and was assigned the code name Hormone.

G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984

Urgent demand for ASW shipboard helo in 1958 won easily by compact Kamov coaxial design, with many other advantages. First prototype, called Ka-20, merely test vehicle for turboshaft engines mounted above fuselage, gearbox and rotors, shown Tushino 1961 with dummy missiles (ASCC reporting name "Harp").

Bill Gunston "The Osprey's Encyclopedia of Russian Aircraft", 2000