In the Missouri Pacific class scheme, the designation P-73 stood for 4-6-2 express locomotives with a driver diameter of 73 inches or 1,854 mm. The class consisted of several batches built by two ALCO works between 1910 and 1926, of which the first were built with drivers of 69 inches and later rebuilt to 73 inches. The first 4-6-2 locomotives ordered by this railroad, which had been the first of this wheel arrangement in the USA, always retained their 69-inch drivers and became class P-69 when the class scheme was later introduced.

The first 39 were built by Schenectady and Brooks in 1910 and 1912, which had drivers of 69 inches, as stated before. They were numbered 6401 to 6439 and originally had a saturated boiler. They were already rebuilt in 1916, where they received a superheater and drivers with a diameter of 73 inches. Their cylinder diameter was increased by two inches, while the stroke was increased by one inch. Later they received 65 square feet of thermic syphons, what increased their direct heating surface to 271 square feet. Some also had Timken roller bearings and an Elasco feed water heater.

The first five locomotives which had 73-inch drivers from the start were built by Schenectady in 1921. They also had a superheater from the start, but still a grate area of only 49.5 square feet like the first ones. In 1924 and 1925, the same ALCO works built 29 which had a grate of 67.8 square feet and were fired with oil. After the Missouri Pacific had bought the International-Great Northern and the St. Louis, Brownsville & Mexico, Brooks built five and six locomotives for these two railroads which were nearly identical to the last batch. All these batches were retired between 1948 and 1954.