When the Spruce Goose was still just a twinkle in Howard Hughes' eye, Russia was quietly constructing the largest propeller-powered plane to ever leave the ground.
The Kalinin K-7 was designed by the Russian Army in the early 1930s as an experimental heavy bomber aircraft that could also be used to transport civilians. Since jet engine technology hadn't yet been invented (Air Commodore Sir Frank Whittle did so single-handed in the late 30s), the K7 relied on conventional prop engines -- and lots of them. It employed 20 engines in total, six propellers along the leading edge of each wing, another pair on the trailing, and some extras sprinkled about as needed.
The K7's unusual twin-boom configuration and under-wing pods, which housed the landing gear and machine gun turrets, actually seated up to 120 passengers inside the plane's 2.1m-tall, 52.7m-wide wings in addition to a crew of 11.
As for military might, the K7 would have been unmatched until the advent of the B-52. According to Gizmag, the Kalinin featured:
...as many as 12 gunner positions, which included an electric cart running along the tail booms to transport gunners to two tail machine guns. It could carry more than 16 tonnes of bombs, 112 fully equipped paratroopers or 8.5 tonnes of parachute drop-able equipment.
Designed by Konstantin Kalinin and built between 1931 and 1933, the K7 was one of a kind. During its first test flight, the super-plane exhibited serious in-flight instability. The plane's frame also contributed to this instability due to its resonance with the frequency of the engines. The K7 did complete seven test flights, proving the plane's ability to reach a 3960m operational ceiling and a 225km/h top speed.
A crash in 1933, which killed 15 people in total and destroyed the aircraft, was widely speculated to be the result of sabotage and political intrigue. That marked the end of the K7's development and preceded the end of Kalinin by five years. In 1938, he was executed as an enemy of the state during Stalinist purges. [Gizmag - Kalinin Wiki - Flying MidShipmen -- Jalopnik -- English Russia]
It Was, According To Some, Developed In Conjunction With Nazi Ufos
K-7 Concept Art (Yes, That's A Nazi UFO)
More K-7 Concept Art
Undated Photo Of The K7 In Testing