The Soviet Union has had a history of ripping off American designs. The Tu-4 "Bull" was pretty much an unlicensed bolt-for-bolt copy of the Boeing B-29 Superfortress. The Su-25 "Frogfoot" was a knockoff of the Northrop A-9. Russia's AA-2 "Atoll" air-to-air missile was pretty much a reverse-engineered Sidewinder.
But the Soviets haven't just kept to swiping combat designs. They've also stolen civilian aircraft data (albeit, one report claims theft of Concorde data used for the Tu-144 "Concordeski" went very wrong). They also apparently knocked off an American transport design.
Boeing YC-14 in flight. This transport was able to haul a main battle tank. (U.S. Air Force photo)
In the early 1970s, the United States considered replacing the C-130 Hercules transport plane. Two contenders engaged in a flyoff. Boeing sent in the YC-14, and McDonnell-Douglas went for the YC-15. Boeing's plane was unusual in that its engines were placed above the wings. This creates what's known as the Coanda effect, and as a result, the plane has great short-takeoff and landing (STOL) performance. TheAviationZone.com notes that the YC-14 had a top speed of 504 miles per hour, and a range of 3,190 miles.
Both the YC-14 and YC-15 did well in the flyoff, greatly exceeding the specs. The YC-14 even proved it could haul a main battle tank! But the need for more strategic airlift meant that neither plane would enter service. The Air Force instead bought what became the C-130H Hercules. Later, a modified version of the YC-15 became the C-17 Globemaster.
A Russian Navy An-72 Coaler. (Wikimedia Commons)
But the Soviet Union also needed a new tactical transport. The Antonov design bureau used the same method that Boeing used to get good STOL performance from the An-72 "Coaler." However, TheAviationZone.com notes that the Coaler has a top speed of only 472 miles per hour, and a maximum range of 2,050 nautical miles. It also can't haul a tank.