Ornithopters were fixed-wing aircraft that flew like birds. They were difficult to create.
The ornithopter was first proposed by Leonardo da Vinci in 1485 when he decided that you could not just attach wings to yourself (Note: Pterosaurs had not been discovered yet.). In the 1800s, attempts were made to build ornithopters. Otto Lilienthal, himself, tried to build an ornithopter but died in a glider accident before he could complete it. No successful manned ornithopter was ever produced until the mid-21st century.
Tech Level: 11-12
Because an ornithopter imitated a bird when it flew, it was more efficient and more maneuverable than a conventional airplane. There ware problems. Conventional materials could not handle the stress. The designs of the wings could not handle the lift and twisting. Computer-optimized designs combined with more advanced materials made ornithopters more practical. As a full understanding of the way a bird flew was reached in the mid-21st century, ornithopters were perfected. They were first used in UAVs. It became possible to combine features of other aircraft into ornithopters to improve performance. That is when they started being used by humans.