As with the Model 25, Massey Harris decided to update the Wallis 12/20 in order to get a few more years out of the Wallis design and avoid making a major outlay for research and design of a new tractor in the Great Depression era of the 1930s.

Wallis had been purchased by Massey Harris in 1928 in order for Massey Harris to become a full line agricultural machinery company. Having a full line to offer farmers was thought to be an advantage at the time. This trend had been started in the early 1900s by IHC which, partially as a result of building and offering for sale every machine likely to be needed by a farmer, was very successful. Other machinery companies realized that they also needed to offer a fairly comprehensive line of machinery in order to keep their customers from going elsewhere to obtain some machine and in doing so, fall into the hands of a competing machinery company.

Massey Harris continued on selling the Wallis line of tractors under the Wallis name into the 1930s. As the Wallis designs were updated, the new production tractors were badged as Massey Harris and a dark green paint scheme with red wheels applied. The Wallis 12/20 was redesigned in the mid 1930s with the resulting Pacemaker PA entering production in 1936. As with the Model 25 the engine speed was increased to 1200 rpm bringing the Pacemaker PA’s rating to 16/27. The Pacemaker PA used a vertical, overhead valve four cylinder engine displacing 248 cubic inches. While the Wallis designed curved boiler plate frame which combined crankcase, transmission case and frame into a one piece unit was retained, the transmission now offered 4 speeds.  Service brakes were also offered. The factory offered either steel wheels or rubber tires as options.

The unstyled Pacemaker PA was built in 1936 and 1937. A styled version of the Pacemaker PA was introduced in late 1937 and was produced until 1939. About 3000 unstyled PAs and 3000 styled PAs were built during the life of the Pacemaker PA.

The styled Pacemaker PA was offered in a standard configuration and the Twin Power configuration.  The standard configuration was mechanically identical to the unstyled Pacemaker PA. The Twin Power version however offered an operators control which increased the governor setting on the engine allowing the engine to achieve 1400 rpm instead of the standard rpm of 1200. This increased the engine’s power output by 10 horsepower. However this control, when increasing the governor setting, also blocked the transmission in neutral. This was done to prevent the increased horsepower from overloading the tractors drive train. The Twin Power option was only useable when the tractor was performing belt work. As with the styled Model 25, the styled Pacemaker PA was painted in a red scheme with straw yellow wheels.

The Pacemaker PA was offered in a row crop configuration with a tricycle front end.  However in the row crop configuration the tractor was called a Challenger Model CH. The three speed transmission of the Wallis 12/20 was retained in the Challenger Model CH but a PTO was offered. A PTO driven, foot activated implement lift was an option that could be ordered. As with the Pacemaker, the Challenger was first built unstyled then offered in a styled version with a red body and yellow wheels. The standard configuration and Twin Power configuration were also offered in the Challenger as well as the choice of steel wheels or rubber tires.