The first self-propelled Massey-Harris combine harvester was the 16-foot No. 20 introduced in 1937. It was designed for the Argentine market. It had a 6-cylinder gasoline engine, 4 gears, a 94 cm wide threshing cylinder, and a 21-bushel grain tank. A smaller No. 21 was planned for America, which went into production in 1941 and was the world's first commercially successful self-propelled combine harvester. The first 21s were made of stainless steel and were unpainted. However, material shortages forced a switch to regular steel.

The No. 21’s engine was a 6-cylinder gasoline engine mounted above the front axle. The transmission was equipped with a variator, which was adjusted stepwise so that 8 threshing speeds were available. The driver’s controls mainly consisted of a horizontally positioned steering wheel and a vertically positioned reel height adjustment wheel. The cutting width was 12 feet. The cut grain was first moved by conveyor belts to the center of the header. In 1943, the 21A was introduced alongside it, which was equipped with an auger feed instead of belts.

In 1948, the Super 26 was introduced as the successor to the 21, from which the British 726 was developed. The 26 had only 2 gears and 24 speed steps. The header height was adjusted electrically. A smaller American model, the 8- or 10-foot 222, was also introduced in 1948, whose British version was the 722. The Super 26’s bigger brother was the 14-foot 27. In 1952, a new series—70, 80, and 90—was introduced. The French 890 was also developed based on these models.

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