The open spaces of airfields were very vulnerable to attack by airborne troops and it was felt that it was particularly important to defend them effectively. However, conventional defences such as pillboxes and trenches could not be installed without danger to friendly aircraft. At this time a number of private companies contacted the government with their own design ideas.
The Pickett-Hamilton fort was designed by Francis Norman Pickett and Donald St Aubyn Hamilton. Pickett (1887–1957) was an engineer. He graduated from
A friend of Norman Pickett, racing driver Donald Campbell, allowed his workshops to be used to build the prototypes. In early 1940,
Winston Churchill wrote to General Ismay on 12 July 1940 saying: "I saw these pillboxes for the first time when I visited
The most common version of the Pickett-Hamilton fort consists of two cylinders of pre-cast concrete each with one end closed. The slightly smaller of the two cylinders slides into the larger and they are kept apart by small guard rollers on the moving part that engaged with grooves. The structure is buried so that the overhanging top of the smaller cylinder lies flush with the ground. Closed in this position the pillbox is inconspicuous and allows aircraft and other vehicles to safely drive straight over it.
The interior is accessed via a small hatch and rungs built into the structure. To bring it into action a lifting mechanism was used to raise the inner cylinder by about 2 feet 6 inches (0.76 m) thereby revealing three embrasures. A crew of two men could then operate the fort as a pillbox.
Initially, the lifting mechanism consisted of a standard 8-ton aeroplane jack that took three minutes to raise the fort. This was soon replaced with a pneumatic ram that was based on a system originally intended for agricultural use. The pneumatic system operated with compressed air stored in cylinders: this allowed the fort to be raised and lowered quickly when speed was essential. A hand pump was also provided to raise the fort for daily maintenance or as a backup method.
An alternative design used counterbalance weights to raise the fort. This allowed the fort to be raised by the physical strength of the garrison. This design had two access hatches and, with a slightly larger underground chamber and the elimination of the central pneumatic ram the fort could have a crew of four men. Under consideration in late 1940, this alternative design was not used in significant numbers and only about a dozen were installed.
The cost of construction was about £240 (equivalent to £11,800 in 2017