Staff Writer (Updated: 2/23/2015):
It goes without saying that fire had always maintained a prominent place on the battlefield even dating as far back to ancient times. It was readily available and the only issue revolved around its adequate delivery onto the heads of enemy formations. "Liquid fire" was nothing new by the time of World War 1 and the Germans used such a flame-spewing weapons to good effect in the trenches against their French enemies. While these systems were large and cumbersome components, they instilled much fear against their intended targets in the fire zone and served as a tremendous psychological presence nonetheless - fire, it seemed, had a way of motivating any living thing to move from its held ground. By the time of the 1930s, the Germans had more or less perfected a man-portable backpack flamethrower (the "Flammenwerfer") that saw good use from it, leading to an ever-growing list of improved forms. It was only a matter of time that the Allies followed suit and developed their own serviceable models. For the Americans, the M1 became such a development - itself being loosely based on the original German design.
When the M1 was pressed into evaluation service by 1941, it was quickly shown to have some reliability issues in both design and operation. The system was hardly robust enough for the rigors of the battlefield and the ignition system - relying on hydrogen being ignited by the spark of a battery - often failed its users to the point that soldiers would use whatever means necessary to ignite the flame gun - burning bits of paper, cigarette lighters etc... The M1A1 was unveiled as an improved, more robust form in 1943 but the system still had a ways to go. Additives were now being added to the fuel stores to produce a "thicker" stream compound, increasing the weapon's range and damage cone. The weight was further "lightened" from 70lbs to make for a more portable system at 65lbs. However, the ignition system originating in the M1 remained unchanged in the M1A1, leading to some of the same problems during combat use.
Design of the M2 began in 1940 and continued on into 1941. With extensive use of the M1 and M1A1 systems, the Chemical Warfare Service - the group responsible for design and delivery of the original flamethrowers - used this experience to develop a more refined weapon system. The prototype came under the designation of "E3" and formed the basis of a new line of more robust and reliable flamethrowing systems. The experimental E3 was eventually accepted into service as the "Portable Flame-Thrower M2-2". The weapon system eventually entered service in 1943 and succeeded both the M1 and M1A1 when numbers made it possible. However, where it was not so, Army and Marine personnel continued use of the M1 family.
The M2 retained the same thickened fuel format as the M1 family but the biggest addition was the new cartridge-based ignition system. The old battery-actuated spark system was dropped from the design, instead replaced by a new five-shot, revolver-type magazine fitted to the end of the flame tube. Each revolver well held an ignition cartridge (for a total of five possible ignitions before reloading). The new system proved much more reliable under combat conditions than the original method, requiring reloading only after all five cartridges had been spent.