The RK.25 was noted by contemporaries for its speed, aerodynamic cleanness, its use of new light alloys and, with a range of 2,000–2,400 km (1,200–1,500 mi; 1,100–1,300 nmi), its suitability for long distance touring.

Structurally, the wings were wooden, each with two longerons which were joined into a box spar by plywood skin out to three-quarter span; this appears dark in the photograph above.

[2] The RK.25's upright Cirrus engine was neatly cowled in weight-saving elektron, following the upper cylinders to minimise their obstruction of the pilot's forward view.

[2] The fuselage had a welded steel tube structure with fabric-covered sides and underside and a curved top which continued the narrow upper engine fairing dorsally.

[2] The second RK.25 (registered D-1701) was fitted with a Cirrus III engine,[3] the improved cylinder head cooling of which[4] required a slightly wider and raised cowling.

This had an enclosed cabin with large side and upper glazing panels, inboard Ksoll slotted, camber-changing flaps and independent, faired undercarriage legs.