The 3.7 cm Flak 18/36/37/43 was a series of anti-aircraft cannon produced by Nazi Germany that saw widespread service in the Second World War. The cannon was fully automatic and effective against aircraft flying at altitudes up to 4,200 m. The cannon was produced in both towed and self-propelled versions. Having a flexible doctrine, the Germans used their anti-aircraft pieces in ground support roles as well; 37 mm caliber guns were no exception to that. With Germany's defeat, production ceased and, overall, 37 mm caliber anti-aircraft cannon fell into gradual disuse, being replaced by the Bofors 40 mm gun and later, by 35-mm anti-aircraft pieces produced in Switzerland.
The original 37 mm gun was developed by Rheinmetall in 1935 as the 3.7 cm Flak 18. It had a barrel length of 57 calibers (hence the additional designation L/57), which allowed 4,800 m (15,700 ft) maximum ceiling. The armour penetration was considerable when using dedicated ammunition, at 100 m distance it could penetrate 36 mm of a 60°-sloped armour, and at 800 m distance correspondingly 24 mm. It used a mechanical bolt for automatic fire, featuring a practical rate of fire of about 80 rounds per minute (rpm). The gun, when emplaced for combat, weighed 1,750 kg (3,860 lb), and complete for transport, including the wheeled mount, 3,560 kg (7,850 lb).
The Flak 18 was only produced in small numbers, and production had already ended in 1936. Development continued, focusing on replacement of the existing cumbersome dual-axle mount with a lighter single-axle one, resulting in a 3.7 cm Flak 36 that cut the complete weight to 1,550 kg (3,420 lb) in combat and 2,400 kg (5,300 lb) in transport. The gun's ballistic characteristics were not changed, although the practical rate of fire was raised to 120 rpm (250 rpm theoretical). A new, simplified sighting system introduced the next year produced the otherwise-identical 3.7 cm Flak 37. The Flak 37 was known as 37 ITK 37 in Finland.
The Flak 36/37 were the most-produced variants of the weapon.
In 1938, the Kingdom of Romania acquired the license to locally produce 360 guns, officially known as "Tun antiaerian Rheinmetall calibru 37 mm model 1939" ("37 mm Rheinmetall anti-aircraft cannon model 1939") at the Astra factory in Brașov. By May 1941, 102 had been built, and by October 1942, the production rate was 6 pieces per month.
In June 1944, the Germans proposed an anti-aircraft version of the Romanian Mareșal tank destroyer, armed with twin 37 mm anti-aircraft guns, a design which was named Flakpanzer Mareșal. However, this proposed design never left the drawing board.
The 3.7 cm Flak M42 was the marine version of the 3.7 cm Flak 36/37 and used by the Kriegsmarine on surface ships and as M42U on Type VII and Type IX U-boats. The 3.7 cm Flak M42U used several types of mounts and entered service in autumn 1943.
The LM 42U mount was developed specifically for the 3.7 cm Flak M42U. It was manned by a 3-man crew, with a fourth man operating as the loader.
The LM 43U mount was the final design of mount used on U-boats. It was a further improvement on the LM 42U. The LM 43U was only known to be installed on these U-boats (U-249, U-826, U-977, U-1023, U-1171, U-1305 and U-1306).
The twin mount was based on the LM 42U design, in which the 3.7 cm Flak M42U guns were mounted side by side. This was one of the best AA weapons used by Nazi Germany's Kriegsmarine during World War II. It was mainly used on the Type IX as it was rather heavy for the Type VII U-boats.
The 3.7 cm Flak 43 was a dramatic improvement over older models. A new gas-operated breech increased the practical firing rate to 150 RPM, while at the same time dropping in weight to 1,250 kg (2,760 lb) in combat, and 2,000 kg (4,400 lb) in transport. It was also produced in a twin-gun mount, the 3.7 cm Flakzwilling 43, although this version was considered somewhat unwieldy and top-heavy.
Many Flak 37s were mounted on the ubiquitous Sd.Kfz. 7 half-track vehicle, or later the schwere Wehrmachtschlepper (sWS), but the newer Flak 43 was almost always used in a mobile mounting. Most famous of these were the converted Panzer IVs, first the "interim" Möbelwagen, and later the Ostwind.
Large-scale production did not start until 1944 and some 7,216 had been produced by the end of the war (Zwillings included, each counted as two guns).
The closest Allied counterpart to the 37 cm Flak series was the 40 mm Bofors L/60, which was designated the "4 cm Flak 28" in German service. The Bofors fired a larger shell of 900 grams (32 oz), as opposed to around 650 grams (23 oz), at a slightly higher muzzle velocity of around 880 metres per second (2,900 ft/s) as opposed to just under 800 metres per second (2,600 ft/s). This gave the Bofors an effective ceiling of about 4,938 metres (16,201 ft) compared to 4,200 metres (13,800 ft) for the Flak series. Firing rates for the earlier models were similar, although the Flak 43's improved rates beat the Bofors. The most notable difference is the size and weight of the two weapons; the Bofors weighed just under 2,000 kilograms (4,400 lb) and required a two-axle trailer, while the earlier Flak models weighed 1,550 kilograms (3,420 lb) on a single-axle mount, and the Flak 43 reduced this even further to 1,250 kilograms (2,760 lb).
The 37 mm automatic air defense gun M1939 (61-K) was the closest Soviet counterpart, firing a shell very similar to the Flak from a gun of generally similar weight and size. The 37, however, had a much lower cyclic rate, averaging around 80 rpm. The US's 37 mm Gun M1 was similar to the Flak as well, but was considered inferior to the Bofors and saw relatively limited use. The Japanese had nothing similar, their largest AA autocannon being the Type 96 25 mm AT/AA Gun. The Italian counterpart was the Breda Cannone-Mitragliera da 37/54..