Dr. Albert Taylor and Leo Young, scientists at the US Naval Aircraft Radio Laboratory, make the 1st successful detections of objects by "radio observation". They use wireless waves to detect objects not visible due to weather or darkness. This insight leads to the advent of radar.
The theory behind radar was first put forward by Christian Hulsmeyer in 1904 and famed inventory Nikola Tesla worked to develop a system of detection in 1917.
The British were the first to fully exploit radar as a defence against aircraft. Robert Watson-Watt demonstrated a working prototype in 1935.