British pop-up and mobile pillbox proposals

What other unusual inventions or ideas have you found in the archives?

Various pop-up pillbox mechanisms and proposals for tractors covered in concrete stand out among British invasion-defense ideas.

Several British proposals used pop-up pillboxes with different mechanisms for raising them into position. Tractors covered in concrete offered another way to create mobile pillboxes, and the RAF used concrete-covered tractors for airfield defense in World War II. These ideas belonged to a recurring debate about fortifying the British Isles. The expense repeatedly prompted the argument that Britain should put the money into its fleet. Fixed defenses could still have a local purpose around sites threatened by airborne attack.