From KSAMC's (Kharkov State Aircraft Manufacturing Company) history page:
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And there is another quite unique project that is worth specific mentioning. It is the story of K-7, which chronologically fits into this same period and yet should be mentioned separately because of the significance of the events associated with it. Back in those years building of giant airplanes had become the aviation manufacturing priority, and Kharkov aircraft manufacturers did not stand aside. In 1929 Kalinin's design bureau completed the design project of the four-motored passenger aircraft K-7, powered by BMW engines of 1000 hp each. The K-7 was a gigantic elliptical wing of the thick profile with the impressive wingspan of 53 meters and wing area of 452 square meters. The passenger modification of K-7 was intended for carrying 128 passengers and claimed the flying range of 5,000 km. The design bureau had also planned to build a VIP configuration of K-7 that featured eight luxurious compartments for eight sleeping berths each, with a panorama windows on the floor, and a comfortable clubroom with couches, galley, bar and radio compartment in the center wing section. The estimated speed and carrying capacity of K-7 kindled the interest of high-ranked military command: as a strategic bomber, K-7 could carry up to 10,000 kg of bombs, its size allowed to install cannons, and the overall layout had made the airplane exceptionally comfortable and safe "flying fortress". The troop-carrying modification of K-7 provided for transporting 112 fully equipped paratroopers.
Problems of construction were many, and in process of building the prototype of the giant K-7 Kharkov design bureau and HAZ encountered them all the time. The design constantly required improvements, the production of materials that the aircraft was built of had just started, the size of the aircraft was way too big for existing manufacturing areas, and welding presented a great problem as well. And yet, these problems were successfully overcome, and having worked for thousands of exhausting hours, Kharkov aircraft manufacturers had created a giant that boggled the imagination. On August 8, 1933 the aircraft was taxied to the runway. On August 19 K-7 made its first flight at the altitude of 5 meters that lasted just a few seconds. Each time test pilots revealed serious problems, vibrations of aircraft parts, etc., and designers and manufacturers made more and more changes to the construction. Thus, the tail of the aircraft was redesigned in just a few days, stabilizers were moved closer to controls, aircraft parts were urgently strengthened.
At 6 a.m. on August 21, 1933 seven crew members took their positions inside K-7. Kalinin himself occupied the seat of the co-pilot. After a few taxis the aircraft smoothly took off. Soon after that the control cable of the servo-assisted steering gear had broken, but the pilot successfully made a circle over Kharkov and landed on the factory airfield.
Three weeks later, after a number of improvements had been made to the aircraft construction, test flights resumed. The second flight took place on September 22, 1933, and the official HAZ flight testing program began. Test flights rarely went without problems and each time revealed more defects that were to be fixed.
The eleventh flight of K-7 turned out to its last one. On November 21, 1933 the aircraft was to be tested again before going to Moscow, and the test pilot got the assignment to measure the maximum speed of the aircraft for the second time (the speed had already been measured the day before but the ground team made a mistake that had cast doubt upon the measurement results). According to the flight plan, K-7 was supposed to take off, fly to the south west of Kharkov (Rogan' district), approach the "measuring kilometer" at 1,000 m altitude, then lower to 100 m and pass the speed base of 1,250 m three times at the maximum speed. At the third run the aircraft approached the "ruler", descended and suddenly dived and hit the ground killing fifteen people of twenty that it had taken on board. The fire started instantly finishing off the aircraft wreck.
Later on the government commission of the most prominent aviation experts summoned for investigating the cause of this catastrophe had come to the conclusion that K-7 crashed because of the deformation of aircraft empennage caused by flatter. At that time there was no solution to that problem, and aviators could not fight the flatter, but considering the success of the previous test flights, the government decided to build two more aircraft of this type. However, considerable changes in the life of Kalinin's design bureau and Kharkov aircraft manufacturing company had already begun to take place, and after the design bureau had been transferred to Voronezh, the construction of the new K-7 was moved there as well. Kharkov aircraft manufacturing company was renamed into "Factory 135" which obviously meant switching to the production of military equipment. The crash of K-7 was neither forgotten nor forgiven, and in 1938 Konstantin Kalinin was arrested, accused of espionage and sabotage, and executed by NKVD.