Replacing the aging 3-inch gun as the staple of Army heavy antiaircraft artillery at the dawn of the war, the 90mm gun went on to earn a well deserved place among the finest artillery pieces fielded by the Allies in World War II.  Intended to meet the threat posed by aircraft capable of flying faster and higher, work on the 90mm began in earnest in 1938.  The 90mm gun M1 was standardized in March 1940.  An M1A1 version followed that added a small loading tray to the breech ring and included accommodations for a spring rammer to facilitate faster reloading.  The updated M2 version of the ninety was standardized in May 1943 and sported a redesigned breech that mounted a fully automatic fuze setter-rammer.  This improvement increased the 90mm gun's rate of fire to an impressive 23 to 28 round per minute.

    The initial M1 mount for the ninety was a traditional "spider" design with a detachable platform and one single-axle, dual-wheeled bogie (four wheels total).  In May 1941, the M1A1 replaced the original model as the standard mount for the ninety.  The M1A1 mount was essentially the same as the M1 version, but introduced remote control capabilities that allowed the gun to be aimed by the battery's director through new electric-hydraulic mechanisms.  The M1A1 could also mount the recoil-cocked spring rammer for the 90mm M1A1 gun.

     Field evaluations of the M1A1 mounts spurred development of the enhanced M2 mount, standardized in May 1943.  The M2 was intended to increase the 90mm gun's capabilities in its secondary role against ground targets.  The redesigned M2 featured single-axel front and rear two-tire bogies, folding platforms and armor shielding for the gun crew.  Unlike the earlier mounts, it was not necessary to fully emplace the M2 before firing, enabling the 90mm gun to engage both air and ground targets more quickly.  The M2 mount also depressed to a -10°, allowing the ninety to wrestle with ground troops, armored vehicles and various water craft when necessary.  Early test versions of the M2 mount, notably the T2E1, made it to the field mounting M1 or M1A1 guns while development of the rammer for the 90mm M2 gun was completed.

     Additionally, there was a fixed-mount M3 pedestal designed for the M1 gun.  This heavily armored mount was designed primarily to convert the ninety into an effective anti-motor torpedo boat weapon, although the M3 mount was also able to fully elevate to +80° and engage aircraft by manual or remote control.  After all, the 90mm was still first and foremost an antiaircraft gun.

     All 90mm mounts incorporated a direct fire sighting system for use against ground targets or watercraft.  

     90mm antiaircraft guns normally operated in batteries of four.  All guns in the battery were controlled by a single director, and would thereby target the same hostile aircraft.  Operators could manually traverse and elevate the guns by matching dial pointers on an indicator that received firing solutions from the battery's director.  One soldier horizontally traversed the gun to the proper azimuth, while a second adjusted the ninety's vertical elevation.  The M1A1 and M2 mounts (and the fixed M3) incorporated remote control capabilities that moved the nineties automatically.  Ground targets were engaged using telescopic gun sights.

Lt. Lawrence Lamm stares intently at the indicator of his M1 90mm gun in the Panama Canal Zone. He is using the handwheel to manually lay the gun toward the predicted future position of the enemy aircraft as calculated by the battery's director and transmitted to the indicator.		Compare the elevation controls and mechanisms of the M2 mount with the M1 controls in the photo to the left. The handwheel has been replaced by a crank for manual operation, a new style indicator regulator has been adopted, and provision has been made for remote control of the gun's elevating and traversing mechanisms.

     The gun sections for 90mm M1 and M2 guns in a mobile battery were identical in the number of soldiers, but assignments of the men were slightly different.  The gun commander was a sergeant who was responsible for a gun squad consisting of a gunner (usually a corporal) along with eight other artillerymen.  The gun commander was also in charge of the ammunition squad led by an ammunition chief, who was a corporal supervising four ammunition handlers and a chauffer - the chauffer being responsible for the gun's prime mover.  In a semimobile unit, the ammunition squad would consist of only the ammunition chief and two handlers.  The gun squad and ammunition squad together made up the gun section.

     Due to the around-the-clock nature of war, illness and casualties, full crews were not always available to man the nineties in the field.  There was no official War Department doctrine specifying a minimum crew for the 90mm pieces, so individual units were left to develop their own contingency plans for manning their weapons in adverse situations.

90mm on M1A1 mount bogged down in deep Italian mud.  Just getting the gun there was often difficult.

M4 prime movers towing the guns of a 90mm battery through a French town.  These special 225-horsepower tractors manufactured by Allis-Chalmers had a maximum speed of 35 MPH on level ground.  A good example of a ninety in traveling position can be seen above the Jeep's hood.  Even though these guns were big, they could move.  One First Army gun battalion was reported to have traveled  275 miles, emplaced, then downed unseen enemy aircraft - all within 36 hours.  The M4 tractor could also move heavy field artillery pieces.

     Much has been made of the legendary German 88mm Flak gun's ability to strike fear into the hearts of Allied infantrymen when used as field artillery.  Less known is the American 90mm's similar track record of terrorizing the Wehrmacht to great effect.  Captured German prisoners referred to the ninety as the "Ratschbum" - an onomatopoeic rendering of the sound the gun made when fired at their positions.  The high velocity of antiaircraft shells shot from a ninety enabled the initial rounds to arrive over enemy positions with little audible warning.  Follow-up rounds were delivered at a rapid tempo, placing accurate fire on surprised hostile troops scrambling for cover.  The ninety's high rate of fire easily outpaced the 2 to 6 rounds per minute of a standard field artillery piece.  This enabled antiaircraft batteries to place an effective concentration of fire on ground positions, making the guns invaluable for disruptive interdiction on enemy troop movements or in a neutralizing counterbattery role against hostile artillery positions. 

     Even thought the ninety proved very useful against ground targets, the piece was not designed to be a field artillery gun.  The relatively small effective burst radius of a 90mm AA shell required a higher volume of fire to inflict damage.  However, sustained rapid fire would overheat the ninety and cause the rifling in the gun tube to erode, making the gun less accurate the more it was fired.  Since antiaircraft engagement is quite different from field artillery work, the unique capabilities of the 90mm guns were used to best effect against ground troops when they supplemented their cousins in Field Artillery.

Cross section of the field of fire for a 90mm M1 antiaircraft gun on mount M1A1.  Clearly depicted is the 20º dead arc in the vertical plane and the limiting range of a standard fuze.  The limiting range prevented the projectile from detonating too close to the gun and its crew.  The stepped pattern is a result of the mechanical limitations of the director. Of course, the actual field of fire would encompass a full circle, not shown in this cutaway view.  Note the decrease in range as the gun is elevated.  (Drawing from field manual FM 4-104.)

Antiaircraft artillerymen attempting to stay warm while on watch in the Ardennes.  Their 90mm M1, positioned for antitank employment, is partially covered to protect it from the snow.  With a muzzle velocity near 2,700 feet per second, flat line trajectory and armor piercing rounds, this ninety has more than enough punch to take on any Panzer that may venture down this road.  However, the lack of shielding on the gun's M1A1 mount could expose these men to small arms fire.  Crouching behind the tires of the gun's bogie will offer the soldiers little protection if German infantrymen advance.  Using the 90mm as a front line artillery piece was always a risk.  Many Panzers fell victim to the big AA guns during the Battle of the Bulge.  However, several nineties were lost when the Germans responded with counter fire.

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