The designation Nieuport 18 appears to have been given to more than one aircraft type, including a single engined fighter and a twin engined three-man bomber.
The first of these aircraft appears to have been the twin engined bomber. This was built late in 1915 with the Nieuport designation 21.000. It was powered by two engines, either 110hp Clerget engines or 150hp Hispano-Suiza engines (it is possible that the Clerget powered version was the Nieuport 19). It carried a crew of three, with the pilot between the wings and gunners in the nose and in the middle of the rear fuselage. It had a larger wingspan and wing area than the Nieuport 15 and was given an extra set of wheels in front of the main wheels to guard against the aircraft going over its nose in a crash landing. On 18 February 1916 the Nieuport 18 designation was officially given to a twin-engined three seat aircraft.
The same Nieuport 18 designation appears to have been given to a later single-seat fighter, powered by a 80hp Le Rhône 9C rotary engine and with the same basic design as the Nieuport 11. The main difference was that the V struts slanted outwards on this Nieuport 18, and the undercarriage was wider.
Stats for two engined version
Engine: Two Hispano-Suiza engines
Power: 150hp each
Empty weight: 2,922lb
Loaded weight: 4,079lb