The Lanz Bulldog was a series of tractors manufactured by Heinrich Lanz AG in Mannheim, Baden-Württemberg, Germany. Production started in 1921 with the Lanz HL, and various versions of the Bulldog were produced up to 1960, one of them being the Lanz Bulldog D 9506. John Deere purchased Lanz in 1956 and started using the name "John Deere Lanz" for the Lanz product line. A few years after the Bulldog was discontinued, the Lanz name fell into disuse. The Lanz Bulldog was one of the most popular German tractors, with over 220,000 of them produced in its long production life. The name "Bulldog" became the general term for tractors in some German regions, most notably in Bavaria.

The Lanz Bulldog was built with a single-cylinder, two-stroke Akroyd engine – the so-called Bulldog engine – that was designed by Fritz Huber. The Bulldog engine was installed horizontally, with the ignition device – the hot bulb – facing forward. It has crankcase scavenging, and intake ports instead of valves. Due to its few moving parts – the piston, crank assembly and flywheel, the fuel injection system and oil system are the only parts that move – it was simple to manufacture, operate and maintain. In the Bulldog engine, fuel is sprayed under low pressure onto the hot-bulb ignition device, where the fuel is ignited and gradually undergoes combustion. This makes the Bulldog engine thermodynamically inefficient, but it requires neither a carburettor like an Otto engine, nor high compression like a Diesel engine. It does not require a special fuel to operate; it can burn regular fuels like diesel fuel or petrol, but also a wide variety of low grade fuel oils – even waste oils. This made the Bulldog engine reasonably economical to operate, despite its high fuel consumption. The original Bulldog had evaporative cooling. Later models use a thermosiphon cooler. For starting, the ignition device has to be heated to ignition temperature using a blow torch, then the engine is hand-cranked with the steering wheel.

Late Bulldog engines have a redesigned hot-bulb with direct injection; they were offered with electric glowplugs and an electric starter motor. Lanz sold these as "Halbdiesel" (half diesel) and "Volldiesel" (full diesel) models, albeit that the engine was not a diesel engine. The Bulldog engine was made with various different displacements, with the 4.8 and 10.3 litre versions being the most common ones.