Concurrently, IHC purchased a number of smaller competitors. Parlin & Orendorff (P&O Plow) and Chattanooga Plow were purchased in 1919. Other brand names they incorporated include Keystone, D.M. Osborne, Kemp, Meadows, Sterling, Weber, Plano, and Champion. In 1924 IH introduced the Farmall, a smaller general-purpose tractor, to fend off competition from Ford Motor Company's Fordson tractors. Farmall was a leader in the emerging row-crop tractor segment. Following the introduction of Farmall, several similarly styled "F Series" models were introduced while the original design continued to be produced as the "Regular." In 1932, IH produced their first diesel engine for the McCormick-Deering TD-40 crawler. This engine would start on gasoline and then switch to diesel. Other diesel engines of this era were difficult to start in cold weather and using gasoline allowed the engine to thoroughly warm up first. In 1935, it was used in the WD-40, becoming the first diesel tractor on wheels in North America (the world's first diesel tractor was Germany's Benz-Sendling BS 6 in 1922). Heavy tractors The market for industrial tractors grew in the 1930s. The TD-40, the first of IH's heavy-equipment crawlers, was suited for a wide range of environments. As demand for construction equipment grew, so did the competition. The diversification of the agricultural tractor range into genuine construction equipment whetted appetites for further expansion. In 1937 IH engaged designer Raymond Loewy to revamp its product line and logo. In 1938 the first such model was the TD-65 heavy tractor, later renamed the TD-18. The letter and standard series For model year 1939, Raymond Loewy created the Farmall "letter series" (A, B, BN, C, H, and M) and the McCormick-Deering "standard series" (W-4, W-6, and W-9). For 1941 the MD model was introduced as the first row crop diesel-powered tractor; over a decade later, IH's largest competitor, John Deere, introduced a diesel option on their row crop models. The letter series tractors were updated to the "super" series in 1953 (with the exception of the A, which had become a "super" in 1947, and the B and BN, which were discontinued in 1948). Many of these tractors (especially the largest, the H, M, and W models) are still in operation on farms today. Especially desirable are the diesel-powered MD, WD-6, and WD-9's. The letter and standard series of tractors was produced until 1954 and was a defining product in IH history. In 1947, the