I read a book about WW2 1 and saw a couple of planes with a giant jet engine on top of them, e.g. the Heinkel He 162 Salamander.
I realize jet engines had to have been invented before WW2 for that kind of technology to be used, so when were the first aircraft jet engines around?
1 Aircraft of World War II: 300 of the World's Greatest Aircraft. Chris Chant.