The Army's interest in a 37mm automatic cannon that could be used in an antiaircraft role dates to the period of the First World War.  Legendary gun designer and arms manufacturer John Browning was approached with a request to develop the new weapon.  Browning's cannon was successfully test fired at Aberdeen Proving Ground in the summer of 1921, but by that time the Army had started to recede into a peacetime mentality.  The urgency of war had passed.  The Army courted the new weapon, considering it a good candidate for multiple uses.  However, ultimate commitment to the 37mm gun by the Army was elusive.  Doubts lingered about the piece. 

     During the interwar years, various versions of the 37mm caliber were revisited, but not adopted for antiaircraft use.  Throughout the 1930s, the .50 caliber machine gun remained the largest automatic weapon available to Coast Artillery antiaircraft units.  As the clouds of war threatened, the need for an antiaircraft automatic cannon able to reach beyond machine gun range became critical.  Some weapon was needed to cover the zone between low-flying aircraft and the area reached by the large 3-inch antiaircraft guns.  Thus, more recent refinements of the 37mm antiaircraft cannon were tested and development of fire control devices for the gun commenced.  Finally adopted in 1939, the 37mm antiaircraft gun became the standard intermediate AA automatic weapon in the run-up to America's entry into the Second World War.

     The 37mm automatic gun was soon to have a rival.  Army Ordnance, with the assistance and encouragement of the British, began testing the Swedish-designed 40mm Bofors automatic gun.  The prevailing opinion among the evaluators was that the 40mm presented the Army with a superior option to the 37mm, so the Bofors was standardized in 1941.  However, the 37mm M1A2 gun saw wide use, especially in the Pacific, before eventually being supplanted by the 40mm Bofors.  

     As the towed-mount versions of the 37mm gun began to fade in the shadow of the Bofors, the Army in 1942 creatively decided to utilize the 37mm gun in other ways.  The cannon was paired with twin M2 .50 caliber Browning machine guns in a coaxial combination mount on the M15 halftrack.  This new lease on life for the 37mm gun was impressively successful.  The M15 halftrack brought the 37mm gun to the European Theater as the standard cannon of self propelled AAA battalions.

As the gun pointers peer through their M7 telescopes on the M2 sighting system, the cannoneer on the right replenishes ammo while the soldier on the left extracts an expended 37mm clip.  New clips could be fed while the gun was firing.        Bathed in autumn sunshine, this gun crew from Battery G of the 71st Coast Artillery (AA) is on  maneuvers with a II Corps anti-tank battalion in North Carolina.  The date was November 3, 1941, just a few short weeks before antiaircraft artillery would take on a new importance with the attack on Pearl Harbor.

     The M1A2 model of the 37mm gun was fielded during World War II.  The standalone 37mm was mounted on either an M3 or M3A1 towed carriage.   The earlier M3 carriage sported the M2 sighting system that used a pair of single-power M7 spotting scopes through which the gun pointers observed the target.  Control was manual.  The M3A1 carriage dropped the spotting scopes in favor of direct fire sights, but also added remote control capabilities to allow the gun to be laid in quadrant elevation and firing azimuth by the M5 director.  Double hand cranks replaced the M3's earlier handwheels on this updated carriage.  These carriages could be lowered quickly from traveling position to firing position in thirty seconds or less.

     The M3E1 version of the 37mm mount was a curiosity.  This was a standard M3 carriage that mounted two Browning M2 .50 caliber water-cooled machine guns in combination with the 37mm cannon.  The M3E1 used the same sighting system found on the M3 carriage.  The M3E1 was primarily an experimental model that was classified as a "limited procurement" type.  However, the idea of pairing a 37mm gun with twin .50 caliber machine guns led to the development of the M42 and M54 mounts used on the M15 series of halftracks.

     The 37mm mobile gun section consisted of a gun commander, gunner (and vertical pointer), lateral gun pointer, loader, assistant loader, ammunition relayer, ammunition handler, and a chauffeur (prime mover driver).  A semimobile unit would not have a chauffeur. 

Test firing of an early 37mm gun on M3 mount at Aberdeen.	37mm crew and their gun silhouetted by a winter sun.  The hoses for the cooling system can be seen coiled around the water chest.

     A little known feature of the 37mm gun is its water cooling system.   Even though the cannon was considered to be an air-cooled piece, a water chest (the same model used for .50 caliber machine guns) was attached to the carriage mounts.  For water cooling, one hose clipped to the muzzle and another to a curved brass tube that was connected to the breech.  Both of these hoses fastened to the water chest.  Water was pumped through the barrel by rotating a crank on the water chest until the gun barrel was cool enough to touch, usually after about two or three minutes of pumping.  It was recommended that the gun be water-cooled after 50 to 60 rounds of continuous fire or after 100 rounds fired in bursts over a short period of time.  The gun, of course, could not be fired during this cooling process.

Although somewhat rare, the M3E1 carriage was part of the inspiration that led to the M15 halftrack. The water-cooled .50 caliber M2 machine guns were replaced by the heavy barrel version on the M42 and M54  mounts found on the M15 and M15A1 halftracks. Many planners initially thought the 37mm canon would replace the .50 caliber machine gun for AA use.  As it turned out, the weapons ended up complementing each other.

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