The Kalinin K-4 was an airliner built in the Soviet Union in the late 1920s which was also adapted for use as a photographic survey aircraft and as an air ambulance.
A further development of the K-1, it was a conventional high-wing, strut-braced monoplane with separate enclosed cabin and cockpit.
Kalinin undertook the design to offer a locally produced alternative to pioneering Ukrainian airline Ukrvozdukhput, which was at that time flying Dornier designs.
The structure was of mixed wood and metal construction, but with major assemblies designed in both wood and metal versions, allowing them to be interchanged.
The design also featured a variable-incidence horizontal stabiliser, and the engine mounting was intended to facilitate the ready interchange of different powerplants.
By May 1928, four pre-production machines were being constructed at the Kharkiv Aviation Factory.
While this work was proceeding, Dobrolyot placed an order with Ukrvozduhput for two photographic survey aircraft, a request that the latter undertook to fill with two specially equipped K-4s.
These machines took priority at the factory and differed from the passenger version in having a ventral hatch for one or two cameras mounted on the cabin floor, plus a self-contained darkroom.
The air ambulance version was developed initially for exhibit at the 1928 Berlin Air Show, and examples would later see military service with Soviet forces in Finland during the Winter War.
In summer the same year, Soviet pilots evaluated the K-4 against a Dornier Merkur that was making a promotional tour.
Encouraged by the positive feedback for his design, Kalinin obtained approval for a demanding flight between Kharkiv, Baku, Tbilisi, and back.
Piloted by Mikhail Artemevich Snegirev, a K-4 named ‘’Red Ukraine’’ made the trip between 22 and 24 August.
This same pilot and aircraft would set a new aerial distance record almost exactly a year later, flying from Kharkiv to Moscow to Irkutsk, then back to Moscow and Kharkiv.
Although the original intention had been to reach Vladivostok, Snegirev still covered 10,400 km (6,500 mi) in 73 hours.
