I've already written about breaking the sound barrier, and I will soon address spaceflight, but there are some really important landmarks in the first half of the 20th century that are well worth taking a look at.
Louis Bleriot proved that monoplanes could fly.
Hugo Junkers built a metal monoplane with cantilevered wings.
There is of course Charles Lindburgh and his famous transatlantic flight, and of equal interest to many are the flights both successful and unseccessful of Amelia Earhart.
The Hindenburg disaster fairly well ended the dream of lighter than air transatlantic liners, and is an event that is very interesting to learn about. These events and people will be the focus of several of my upcoming articles. Many people know something about Howard Hughes from the recent movie about him, and I am planning an article about him as well.
Improvements in the power and weight of the internal combustion engine made the transition from planes built of wood and cloth to much less fragile metal planes (the J1) possible, and the jet engine soon followed.