The design was by Dipl-Ing Klaus Holighaus and the prototype first flew in May 1974. The production examples incorporated several improvements in January 1975. The Janus has a glass-fibre monocoque fuselage similar to that of the Nimbus-2 but the cockpit section is lengthened to accommodate the two pilots in tandem with dual controls under a right-hand side hinged one-piece canopy. Landing gear consists of either a non-retractable main wheel (Janus A, B & C) or a retractable main wheel (later models of Janus C and all Ce's) with a drum or disc brake, and a nose-wheel. The two-piece wings have 2° forward sweep on the leading edge, and have camber-changing flaps which are operated between +12° and -7°. The Janus has upper surface airbrakes, and although uncommon the Janus C could also be fitted with a tail parachute.
a motorised version with a Rotax engine mounted on a pylon aft of the cockpit and retracting into the fuselage. The prototype first flew in 1978.
The French developed the Janus as the SCAP–Lanaverre SL-2. (SCAP - Société de Commercialisation Aéronautique du Plessis SàRL) with Lanaverre Industries, It first flew in 1977. The main differences from the Janus A are the provision for water ballast in the wings, a fixed tailplane with elevators, and a more comfortable cockpit.