The aircraft was powered by a single Maybach Mb.IVa water-cooled inline-engine providing 305 hp (228 kW), an engine type usually used to power Zeppelins or Riesenflugzeug like the Zeppelin-Staaken R.VI, received as part of Germany's reparations to Japan after the end of the First World War.
[1][2] The first example of the new aircraft, the K-7A Transport Seaplane was completed in November 1924, demonstrating both good performance and handling.
A further nine K-7As were built by the time production ended in 1927, together with a single example of the K-7B Mail-Carrying Aircraft, a modified version that could be operated either on floats or with a tailwheel undercarriage.
The K-7A proved successful in service, one carrying mail from Fukuoka and Shanghai, China, a distance of over 950 kilometers (590 mi) in May 1926, while the K-7B was used together with the Kawanishi K-10 Transport to operate an airmail route between Osaka and the Japanese occupied city of Dalian in September 1926.
The K-7A continued in use until 1929, when Nippon Koku was forced to be disbanded, with its routes transferred to the government owned Nihon Koko Yuso KK (Japan Air Transport).