The Keystone K-55 Pronto was a mail plane developed in the United States in the late 1920s.

The Pronto was a conventional single-bay, unequal-span biplane design with slightly staggered wings.

The first flight of this service was made by two Prontos, piloted by Leonardo Alvariño Herr and Harold B.

At least one Peruvian Pronto was fitted with pontoons and used to survey the Amazon River for locations suitable for seaplane operations.

In the Colombia-Peru War one Peruvian K-55 flown by Lieutenant Suero was captured by the Colombian Army.