Development of the gun began in 1936, with the contract being awarded to Rheinmetall Borsig. The first prototype gun was delivered for testing in late 1937 and completed testing successfully. The gun weighed nearly 12 tonnes in its firing position, with the result that its barrel had to be removed for transport. Limited service testing showed this was impractical, so in 1938 other solutions were considered. The eventual solution was to simplify the firing platform, based on the assumption it would always be securely bolted into concrete. The total weight of the Flakzwilling twin-gun mount system reached 26.5 tonnes, making it practically impossible to tow cross-country. In the end, this mattered little since by the time the gun entered production in 1942, it was used in primarily static, defensive applications. There were four twin mounts on the fortified anti-aircraftZoo Tower, and they were also on other flak towers protecting Berlin, Hamburg, and Vienna. It is claimed that during the Battle of Berlinthe guns on the Zoo Tower were used successfully to support ground forces, "where the heavy 128 mm shells obliterated Soviet armor, especially when hit from the side". The rush to capture the Reichstag led to dozens of tanks being destroyed. Approximately 200 guns were mounted on railcars, providing limited mobility. The gun fired a 27.9 kg shell at 880 m/s to a maximum ceiling of 14,800 m. Compared with the 88 mm FlaK 18 & 36, the FlaK 40 used a powder charge four times as great. In December 1943, Hitler decided not to introduce the FlaK 40 "During this war".
Variants
12.8 cm FlaK 40
12.8 cm Flakzwilling 40/2 The 12.8 cm Flak 40 ordnance on a static dual mounting with a total weight of 26 tonnes, capable of firing 20 rounds per minute. Used mainly on flak towers. Production started in 1942 with 10 twin sets produced, another eight in 1943, and in February 1945 a total of 34 were available.
120 mm M1 gun, a US gun of almost identical performance
QF 4.5-inch gun: British 113 mm heavy anti-aircraft gun firing slightly lighter shell
QF 5.25-inch gun: British 133 mm heavy anti-aircraft gun firing heavier shell
5"/38 caliber gun: US Navy single/dual purpose shipboard 127 mm heavy anti-aircraft gun firing slightly lighter shell, used on many U.S. Navy ships built during World War II
