The Swedish Navy purchased a number of 2 pounder Pom-Poms from Vickers as anti-aircraft guns in 1922. The Navy approached Bofors about the development of a more capable replacement. Bofors signed a contract in late 1928. Bofors produced a gun that was a smaller version of a 57 mm (6-pounder) semi-automatic gun developed as an anti-torpedo boat weapon in the late 19th century by Finspong. Their first test gun was a re-barreled Nordenfelt version of the Finspong gun, to which was added a semi-automatic loading mechanism.
Testing of this gun in 1929 demonstrated that a problem existed feeding the weapon in order to maintain a reasonable rate of fire. A mechanism that was strong enough to handle the stresses of moving the large round was too heavy to move quickly enough to fire rapidly. One attempt to solve this problem used zinc shell cases that burned up when fired. This proved to leave heavy zinc deposits in the barrel, and had to be abandoned. In the summer of 1930 they began experimenting with a new test gun that did away with controlled feed and instead flicked the spent casing out the rear whereafter a second mechanism reloaded the gun by "throwing" a fresh round from the magazine into the open breech. This seemed to be the solution they needed, improving firing rates to an acceptable level, and the work on a prototype commenced soon after.
During this period Krupp purchased a one-third share of Bofors. Krupp engineers started the process of updating the Bofors factories to use modern equipment and metallurgy, but the 40 mm project was kept secret.
The prototype was completed and fired in November 1931, and by the middle of the month it was firing strings of two and three rounds. Changes to the feed mechanism were all that remained, and by the end of the year it was operating at 130 rounds per minute. Continued development was needed to turn it into a weapon suitable for production, which was completed in October 1933. Since acceptance trials had been passed the year before, this became known as the 40 mm akan M/32. Most forces referred to it as the Bofors 40 mm L/60, although the barrel was actually 56.25 calibres in length, not the 60 calibres that the name implies.
The gun fired a 900 g (2.0 lb) high explosive 40 × 311R (rimmed) shell at 2,960 ft/s (900 m/s).] The rate of fire was normally about 120 rounds per minute (2.0 rounds per second), which improved slightly when the barrels were closer to the horizontal as gravity assisted the feeding from the top-mounted magazine. In practice firing rates were closer to 80–100 rpm (1.3–1.7 rounds per second), as the rounds were fed into the breech from four round clips which had to be replaced by hand. The maximum attainable ceiling was 7,200 m (23,600 ft), but the practical maximum was about 3,800 m (12,500 ft).
The gun was provided with an advanced sighting system. The trainer and layer were both provided with reflector sights for aiming, while a third crew-member standing behind them "adjusted" for lead using a simple mechanical computer. Power for the sights was supplied from a 6V battery.
In spite of the successful development, the Swedish Navy changed its mind and decided it needed a smaller hand-swung weapon of 13 mm-25 mm size, and tested various designs from foreign suppliers. With the 40 mm well along in development, Bofors offered a 25 mm version in 1932, which was eventually selected as the 25 mm akan M/32.
The first version of the 40 mm the Navy ordered was intended for use on submarines, where the larger calibre allowed the gun to be used for both AA and against smaller ships. The barrel was shorter at 42 calibers long, with the effect of reducing the muzzle velocity to about 700 m/s (2,300 ft/s). When not in use, the gun was pointed directly up and retracted into a watertight cylinder. The only known submarines that used this arrangement was the Sjölejonet class boats. The guns were later removed as the subs were modified with streamlined conning towers.
The first order for the "real" L/60 was made by the Dutch Navy, who ordered five twin-gun mounts for the cruiser De Ruyter in August 1934. These guns were stabilized using the Hazemeyer mount, in which one set of layers aimed the gun, while a second manually stabilized the platform the gun sat on. All five mounts were operated by one fire control system.
Bofors also developed a towable carriage which they displayed in April 1935 at a show in Belgium. This mount allowed the gun to be fired from the carriage with no setup required, although with limited accuracy. If time was available for setup, the gunners used the tow-bar and muzzle lock as levers, raising the wheels off the ground and thereby lowering the gun onto supporting pads. Two additional legs folded out to the sides, and the platform was then leveled with hand cranks. The entire setup process could be completed in under a minute.
Orders for the land based versions were immediate, starting with an order for eight weapons from Belgium in August 1935, and followed by a flood of orders from other forces including Poland, Norway, and Finland. It was accepted into the Swedish Army the next year, known as the 40 mm lvakan m/36, the lower-case m indicating an Army model as opposed to the capital M for Navy.
Because of the labour shortages some of the Bofors 40mm factories were opened in Poland.
The Swedish Navy adopted the weapon as the m/36 in hand-worked single air-cooled, and power operated twin water-cooled version. A twin air-cooled mounting, probably hand-worked was also used by the navies of Sweden and Argentina and a twin air-cooled wet mounting was developed for Polish submarines.
The British Army had first examined the weapon when they received a number of Polish-built examples in 1937 for testing, known as the QF 40 mm Mark I (QF standing for "Quick Firing"), or Mark I/2 after a minor change to the flash hider. A licence was acquired and the gun was converted from metric to imperial measurements. They also made numerous changes to the design to make it more suitable to mass production, as the original Bofors design was intended to be hand-assembled, and many parts were labeled "file to fit on assembly", requiring many man-hours of work to complete.
Testing showed that aiming the guns against high-speed aircraft was a serious problem. Although the gun could be trained quickly, aiming accurately while doing so proved difficult. In order to address this, the British introduced a complex mechanical analogue computer, the Kerrison Director, which drove the laying electrically. A three-man team operated the Director simply by pointing it at the target and dialing in estimates for range and various atmospheric conditions. The Director then aimed the guns directly through powered mounts, as the gunners loaded the clips. Backup sights were fitted to the individual guns, replacing the former reflector sight and lead-calculator with a ring-and-post sight known as a "pancake".
In this form, the QF 40 mm Mark III (Mk II was a designation used for a Vickers "pom-pom"), became the Army's standard light AA (anti-aircraft) weapon, operating alongside their 3-inch and 3.7-inch heavy weapons. The gun was considered so important to the defence of England after the fall of France in 1940 that a movie, The Gun, was produced to encourage machinists to work harder and complete more of them. By the end of the war total production from British, Canadian, and Australian factories was over 2,100, while U.S. lend-lease examples added about 150.
In combat it was found that the Kerrison was difficult to set up to use in many situations, as well as making logistics more complex due to the need to keep its electrical generator supplied with fuel. In most engagements only the pancake sights were used, without any form of correction, making the British versions less capable than those used by other forces. Eventually an anti-aircraft gunnery school on the range at Stiffkey on the Norfolk coast delivered a workable solution, a trapeze-like arrangement that moved the pancake sights to offer lead correction, operated by a new crew-member standing behind the left-hand layer. The Stiffkey Sight was sent out to units in 1943, arriving in Canadian units in the midst of the Battle of the Aleutian Islands. A final wartime change to the elevation mechanism resulted in theQF 40 mm Mark XII. They also designed a much lighter two-wheeled carriage for airborne use.
The Army also experimented with various self-propelled AA systems (SPAAGs) based on various tank chassis. Changes to the breech for this role created the QF 40 mm Mark VI, which was used on theCrusader to produce the Crusader III AA Mark I. The main self-propelled version of the Bofors used the gun set on the chassis of a Morris Commercial four-wheel drive lorry. Such guns were used in support of Army divisions to provide swift protection against air attack without the need to unlimber. this was known as the "Carrier, SP, 4x4 40mm, AA (Bofors) 30cwt". They saw service in North West Europe, where six SP Bofors of 92nd (Loyals) Light Anti-Aircraft Regiment, Royal Artillery, landed with the British 3rd Infantry Division on Sword Beach on D-Day to protect the vital bridges over the Caen Canal andOrne River (Pegasus Bridge and Horsa Bridge), shooting down 17 German planes. Later in the campaign, SP Bofors were used extensively for ground shoots as well as in an anti-aircraft role. In British army service the Bofors found a highly specialised role: during the North Africa Campaign at the Battle of El Alamein, they were used to fire tracer horizontally to mark safe paths for units through the German minefields. This practice was further developed during operations in North-West Europe, where bursts of colour-coded tracer were used to define the axis of advance of the different formations in large-scale night attacks.
The Royal Navy also made extensive use of the Bofors. Their first examples were air-cooled versions quickly adapted to ships during the withdrawal from Norway. With the fall of the west in 1940 the Dutch mine-layer Willem van der Zaan brought them their first example of a water-cooled gun on their Hazemeyer tri-axially stabilized mounting. Locally produced examples started arriving in 1942, known as theQF 40 mm Mark IV for use in twin-mounts, or the QF 40 mm Mark V for single mounts. The Navy ran through a variety of versions of the basic Bofors gun over the war, including the Mark VII to Mark XI. The Royal Navy's home-grown light anti-aircraft weapon, the QF 2-pounder gun, also had a caliber of 40 mm, but was referred to as the QF 2 pdr.
The designation of models in Royal Navy service is confused as the gun and its mounting received separate mark numbers. The following mountings were used;
· Mark I: twin mounting based on American design and using American built guns, not widely fitted. Fitted for remote fire control.
· Mark II: quadruple version similar to Mark I
· Mark III: a navalized version of the Army single mounting, hand worked elevation and training.
· Mark IV: a tri-axially stabilized twin mounting copied from, and usually known as, the "Hazemeyer". It had on-mounting fire control, and was usually fitted with Radar Type 282 to provide target range information.
· Mark V: twin mounting, that superseded and eventually replaced the Mark IV, often referred to as the "utility" mounting. This was a simplified, unstabilised mounting based on the American twin mounting Mark I, and was designed for remote fire control.
· Mark VI: a six-barreled weapon feeding from large trays instead of clips and designed for remote control from a dedicated radar-equipped director.
· Mark VII: a single barreled, hydraulically powered mounting that superseded the Mark III and entered service in 1945.
· Mark IX: Mark VII mount modified to electrical power, as the Mounting Mark IX, and in this form saw service in the Falklands War.
The Mounting Mark V (Mark VC for Canadian built examples) for the 20 mm Oerlikon and QF 2 pounder guns was also adopted initially as an interim mount for the Bofors. It was a single barreled mounting with hydraulic power, and was known as the Boffin.
The final British Bofors mounting that saw service was the STAAG (Stabilized Tachymetric Anti Aircraft Gun') which was twin-barrelled, stabilised, and carried its own tachymetric (i.e. predictive) fire control system, based around the centimeter Radar Type 262, capable of "locking on" to a target. This mounting was heavy (17.5 tons) and the high-vibration environment of the gun mounting was poor location for sensitive valve electronics and mechanical computers. STAAG Mark I carried the radar dish over the gun barrels where it was subject to damage during firing, therefore STAAG Mark II shifted the set to the roof of the control cabin. STAAG was ultimately too difficult to maintain in the harsh environment of a warship and was later replaced by the Mounting Mark V with the fire control equipment located remotely, the single Mark VII and ultimately, with the Sea Cat missile.
Visitors U.S. versions
In order to supply both the Army and Navy with much greater numbers of the guns, Chrysler built 60,000 of the guns and 120,000 barrels through the war, at half the original projected cost, and filling the Army's needs by 1943. Over the lifetime of the production, their engineers introduced numerous additional changes to improve mass production, eventually reducing the overall time needed to build a gun by half; most of these changes were in production methods rather than the design of the gun itself.
There were many difficulties in producing the guns within the United States, beyond their complexity (illustrated by the use of 2,000 subcontractors in 330 cities and 12 Chrysler factories to make and assemble the parts). The drawings were metric, in Swedish and read from the first angle of projection, with lower precision than needed for mass production. Chrysler had to translate to English, fix absolute dimensions, and switch to the third angle of projection. Chrysler engineers also tried to simplify the gun, unsuccessfully, and to take high speed movies to find possible improvements, but this was not possible until near the end of the war.
"It should be noted that the USN considered the original Bofors Model 1936 design to be completely unsuitable for the mass production techniques required for the vast number of guns needed to equip the ships of the US Navy. Firstly, the Swedish guns were designed using metric measurement units, a system all but unknown in the USA at that time. Worse still, the dimensioning on the Swedish drawings often did not match the actual measurements taken of the weapons. Secondly, the Swedish guns required a great deal of hand work in order to make the finished weapon. For example, Swedish blueprints had many notes on them such as "file to fit at assembly" and "drill to fit at assembly," all of which took much production time in order to implement. Thirdly, the Swedish mountings were manually worked, while the USN required power-worked mountings in order to attain the fast elevation and training speeds necessary to engage modern aircraft. Fourthly, the Swedish guns were air-cooled, limiting their ability to fire long bursts, a necessity for most naval AA engagements. Finally, the USN rejected the Swedish ammunition design, as it was not boresafe, the fuze was found to be too sensitive for normal shipboard use and its overall design was determined to be unsuitable for mass production. US manufacturers made radical changes to the Swedish design in order to minimize these problems and as a result the guns and mountings produced in the USA bore little resemblance their Swedish ancestors."
The United States Navy's Bureau of Ordnance purchased a twin-mount air-cooled example directly from Bofors, which arrived in New York on 28 August 1940. During that month another Dutch ship, the van Kinsbergen, demonstrated the Hazemeyer mount to Navy observers. The gun was quickly chosen as the Navy's standard anti-aircraft weapon, and the Navy secretly imported a set of Imperial designs from England and started production illegally. A formal contract with Bofors followed in June 1941. The resulting Mark 1 and Mark 2 weapons were intended for the left and right side of a twin mount, respectively, and were adapted by Chrysler for water cooling. After the war, the 3"/50 caliber gun Mark 27 mount began to replace the Bofors, because the "VT" proximity fuse would not fit a 40mm projectile.
The Navy's satisfaction with the weapons was demonstrated by their practice of telegraphing Chrysler Corporation with the serial numbers of guns when they shot down an aircraft.
The United States Army had recently introduced a 37 mm gun of their own design, but found it to be of limited performance. Six British Bofors were imported for testing, along with the Kerrison Directors, and proved to be superior in all areas.
In U.S. Army service, the single mount Bofors was known as the 40 mm Automatic Gun M1. The U.S. version of the gun fired three variants of the British Mk. II high-explosive shell as well as the M81A1 armor-piercing round, which was capable of penetrating some 50 mm of homogeneous armor plate at a range of 500 yards.
The dual version of the gun was mounted on an M24 Chaffee tank chassis and was called the M19 Gun Motor Carriage.
The Wehrmacht used a number of Bofors guns which had been captured in Poland and France. The Kriegsmarine also operated some guns obtained from Norway.
In German naval use, the gun was designated the 4 cm Flak 28, and was used aboard the cruisers Admiral Hipper and Prinz Eugen toward the end of the war.
Japan captured a number of Bofors guns in Singapore and put them into production as the Type 5.
Both Japan and West Germany continued to use the Bofors gun throughout the Cold War.