McDonnell Douglas MD-81 UHB Demonstrator
Very suprised this gem hasn't been modeled before.
In the mid-1980's the high price of fuel led to the investigation of alternatives powerplants. The unducted fan concept was a hybrid between a jet and a standard turbo-prop engine that promised excellent fuel efficiency with the speed and power of jets. Both Boeing and McDonnell Douglas prepared to produce lines of new aircraft with the new technology, both flying engine test beds with a UDF engine on one side.
Unfortunately the the UDF engines were significantly more expensive than the equivalent jet engine, and the Western UDF engines were never put in to production.