Fundamentally a variant of the Nieuport 17 in which the 110 hp Le Rhône 9J gave place to the 80 hp Le Rhône 9C, the Nieuport 21 was built in considerable numbers, and, because many were fitted with a horseshoe-form engine cowling, it was frequently mistaken for the Nieuport 11. It had the larger wings and bracing geometry of the Nieuport 17, however, and in RNAS service (at least five being acquired) it was known confusingly as the Nieuport 17B.
The Nieuport 21 was also supplied to Russia, and, from September 1917, 181 were acquired by the US Air Service for training duties. At least one was tested with the 90 hp Le Rhône 9Ga engine.
