The Keystone K-55 Pronto was a mail plane developed in the United States in the late 1920s. It was a conventional single-bay, unequal-span biplane design with slightly staggered wings. The pilot sat in an open cockpit, in tandem with a second cockpit that could carry two passengers side-by-side. The fixed, tailskid undercarriage had divided main units.A number of these aircraft were purchased by the government of Peru.