It was used by the United States Army Air Corps with the designation PT-11 and the United States Coast Guard under the designation N4Y.

The Model 21 was an aerodynamic cleaned up version of the Model 12/PT-3, one of the distinguishing features being curved instead of angular tail surfaces.

The aircraft was a single-engined biplane with fixed tailwheel landing gear and accommodation for two in open cockpits.

Designated the PT-11 by the United States Army Air Corps it progressed through a number of trial variants but was not built in large numbers.

Data from "The Illustrated Encyclopedia of Aircraft (Part Work 1982-1985)" Orbis PublishingGeneral characteristics Performance