The155mm-armed M109 Self-Propelled Howitzer (SPH) was developed concurrently with the 105mm-armed M108 SPH. Both shared the same hull superstructure and turret design and were differentiated primarily by their choice of armament. The M108 was eventually given up in the long run in favor of the more powerful M109 offering to which the M109 has since enjoyed an exceedingly long operational service life thanks, in part, to a strong design and modernization programs. Its endorsement by the U.S. Army and USMC - and its subsequent combat use during the Vietnam War (1955-1975) - led to its wide acceptance by other global land armies aligned with the United States. Production of both the M108 and M109 systems began in 1962 though manufacture of the M108 ended the following year. The M109 continued in production up until 1969 and was manufactured under several related brand labels: General Motors (Cadillac Motor Car Division), General Motors (Allison), and Chrysler Corporation - interestingly all from the same Cleveland facility. Bowen-McLaughlin-York added additional production in 1974 to meet new demand.
Initial M109 production vehicles were fitted with the T255E4 short-barreled main gun which proved effective but led to excessive wear-and-tear due to the propellant charges in use. This prompted a slight revision of the design which introduced a longer-barreled gun tube in the XM185. The existing M109 fleet was then converted to the new M109A1 standard beginning in 1972 with operational levels reached the following year. During 1974, more M109A1s were built to strengthen existing stocks, these by Bowen-McLaughlin-York, and designated M109A1B.
As completed, the M109 was a conventional SPA form by modern standards. It fitted its powerpack at front-right with the driver at front-left in the forward hull. The turret was placed over the rear section of the vehicle with the large main gun fitted into the forward panel. A cupola was afforded to the commander's position to which a 0.50 or 0.30 caliber machine gun could be installed for air/local defense. The vehicle sat atop a track-and-wheel arrangement which included seven double-tired road wheels to a hull side. The drive sprocket was at front with the track idler at rear while no track return rollers were used. A small door at the rear of the hull allowed for crew entry exit as did side panels, roof hatches and a hatch over the driver's compartment. Main guns featured massive muzzle brakes and were clamped to the hull when traveling. Despite the 155mm caliber, there proved little barrel overhang. The operating crew numbered six - driver, commander, two gunners and two loaders. Turret traversal was a full 360-degrees.
After a period of in-the-field use, more revisions were ordered which included a larger turret bustle which accepted more onboard ammunition storage. New gun mounting hardware was also installed and the floatation equipment seen in original production models was dropped. With these changes in place, the U.S. Army adopted the M109A2 standard and 823 x A2 models followed from 1976 into 1985. Existing M109A1 and M109A1B models were all modified to the A2-standard and these became M109A3.
The next major revision of the M109 line occurred during the middle part of the 1980s prior to the end of the Cold War. As the threat of nuclear war had not subsided, it was thought prudent to provide M109 crews with some form of local protection from Nuclear, Biological, and Chemical (NBC) agents and, thusly, an NBC suite was added to the M109. Existing M109A2 and M109A3 marks then graduated in this fashion to become the M109A4. The upcoming M109A5 introduced the M284 series 155mm main gun which replaced the original's M185 model and a 440 horsepower diesel engine was brought along. Expectedly, maximum range of the gun was increased as was performance of the vehicle. M109A5+ signified M109A5s with updated Fire Control Systems (FCSs) and other more subtle internal changes.
After Operation Desert Storm (1991), the M109A6 "Paladin" entered service in 1992 as a wholly improved M109 mark - which many regard as the ultimate evolution of the series. The A6 model improved armor protection for the crew and systems alike and further increased ammunition stocks aboard. An automatic FCS was introduced which assisted in gun laying and improved accuracy. The NBC suite and crew comfort was further addressed while the gun received additional attention. Unassisted engagement ranges now peaked at around 18.6 miles while Rocket-Assisted Projectiles (RAPs) added additional downrange reach. By this time the crew had been reduced to four from six.
M109s were resupplied in the field with additional ammunition and charges through the M992 vehicle. This development was essentially a turret-less M109 though with a fixed superstructure and three crew. The vehicle typically followed M109s into action and supplied the needed munition types on call, the shells being passed through openings at the rear of the M109.
The M109 proved an export success on the global stage, one of the most successful armored vehicles of the Cold War and one of the best American SPG/SPG systems since the close of World War 2 (1939-1945). Operators ranged from Canada to South America, Europe and the Middle East. Chances are those nations fielding an SPG during the Cold War managed a stock of either the Soviet equivalent machine or the American-made M109. Many remain in service today - such has been the effectiveness of the vehicle series in combat.
Combat service has taken the M109 through the Vietnam campaign, the Yom Kippur War (1973), the Iran-Iraq War (), the Gulf War (1991), the recent U.S.-led Iraq campaign (2003-2011), and the 2006 Lebanon War. The British Army relied on the M109 for a time until succeeding these units with the locally-designed and developed AS-90 system appearing in 1993.
At one point, the XM2001 "Crusader", detailed elsewhere on this site, was intended as the American successor for the M109 line. This promising program was eventually terminated in May of 1992 - signaling an extended service life for existing M109 vehicles.