WWII enlisted men's woolen trousers can essentially be divided into two lineages: Those with pre-war origins that were designed along the lines of commercial men's slacks and those designed during the war with field use as their primary function. Woolen field trousers were new designs, first coming on to the scene in 1943, and were developed with several new features to provide soldiers with an improved garment to wear while campaigning. The field trouser design completed the new wool field uniform, with other main component being the olive drab wool field jacket. They were also to be used as part of the newly adopted "layering principle", providing warmth underneath the wind and waterproof olive drab cotton field trousers.

Field trousers were originally developed in 22 ounce serge wool for warmth. However, it was soon realized that the weight of the 22 oz material was too heavy for general issue purposes, and would be too difficult to procure in sufficient quantity to meet demand. As a result, 18 ounce serge wool became the standard material for field trousers, while the 22 ounce version (Specification 372B, 30 June 1943) continued in limited production to be issued in cold climate conditions. Other distinguishing features of the original wool field trouser design included canted front pocket openings for easier access, a closure for the left rear pocket, buttons and straps for suspender use, and ankle gathering tabs for blousing over the combat boot. The general issue 18 ounce field trousers (specification 353A, 12 April 1944) retained most of the features of the original design with the exception of the ankle gathering tabs, and the straps for the hook type suspenders.

Production of the old style "light shade" or "mustard color" service trousers ceased in November 1943 as development of the new field trousers progressed. This left large stocks of the old fabric to be used up. The new field uniform consisting of trousers and jacket were to be produced in the darker olive drab shade 33, which offered better camouflage properties in the field. (point here for color comparison)
Color comparison: Pictured to the left is the Regular Light-Shade Olive Drab Serge Wool Trousers (Spec. 8-83B, of 9 November 1937) and to the right the Special 18 ounce Olive Drab Shade 33 Serge Wool Field Trousers (Spec. PQD 353B, of 21 November 1944). The Army procured massive quantities of light shade serge wool material for service trouser production. When the enlisted man's wool trousers were redesigned for field use in 1943, a darker color was chosen that offered better camouflage properties. Excess stocks of light shade material continued to be used up in field trouser production through early 1945.

Early on, this resulted in field trousers being produced in the two different colors. To distinguish the two types, a new system of stock tariffs was introduced: 55-T-350 to identify the light shade and 55-T-355 to identify the olive drab shade 33 trousers. In addition to using up stocks of light shade fabric, the Army also utilized available stocks of 18 oz. melton and kersey wool in the production of field trousers.

It was a common wartime practice to maximize available manufacturing capacity to meet demanding production requirements. This meant that trouser manufacturers were not required to retool, change production methods, or assembly lines to fill orders as long as they could meet the majority of the specification requirements. As a result some field trousers were manufactured with the old style vertical slit front pockets, some were constructed using a separate piece waistband, and rear pockets could be constructed using either a button and hole arrangement or a flap and concealed button. Color, fabric, and construction differences resulted in many variations of the field trouser being produced throughout the war period.
(point here for variation example)
Differing plant equipment and manufacturing methods accounted for variations in finished clothing products. Pictured are two examples of the specification 353B field trousers. The one on the left, produced by Merit Clothing Company, has a waistband built into the rise and has canted front pockets. The example on the right, produced by the Goodimate Company, has a separately constructed and attached waistband and vertical slit front pockets.

With specification 353B of 21 November 1944, the woolen field trousers were standardized to the darker Olive Drab Shade 33 color. The stock number tariff indicated this by the 55-T-355 designation, and the specific color was also identified directly in the nomenclature label. Although the color was standardized, minor manufacturing variations continued in front and rear pocket design and waist construction. But, essentially the woolen field trouser of this time appeared as an olive drab shade 33 trouser with a gas flap behind the front fly, was outfitted with suspender buttons, had canted front pockets, and a button and hole closure on the left rear pocket. The 18 oz. woolen field trousers would remain in this basic configuration until October 1945 when a new pattern was drawn up calling for both rear pockets to have flaps with a concealed button feature.