On September 20, 1948, the first Soviet-made mass-produced helicopter Mi-1 made its first flight. It entered service in 1950 and was produced for 16 years. More than 1,000 were built in the USSR and 1,594 in Poland, as SM-1. The Mil Design Bureau became one of the world's leading developers of helicopters, with particular expertise in heavy-lift helicopters. Ninety-five percent of the helicopters in the former Soviet Union were built to Mil designs. Mil merged with Kamov and Rostvertol to form Russian Helicopters in 2006. See the most unique Mil helicopters.