The US military’s Defense Advanced Research Projects Agency (DARPA), has been working on developing an ornithopter* since 2005. The robotic humming bird, being developed under the Nano Air Vehicle (NAV) program, can fly at a speed of 11 miles per hour. While not on a par with its biological counterpart in terms of speed, it weighs a mere 20 grams.
The engineers had to overcome a plethora of design challenges to meet the steep specifications; specially in developing the wing structure, propulsion and control actuators. This video demonstrates the advancement of this project over the course of time.
The part where the robot crashes multiple times, almost made me cry. Almost. That is if you can overlook the fact that these are being built for nefarious purposes. They might be used as remote assassins in future!
*ornithopter = Heavier-than-air craft that is propelled by the flapping of wings