Oshkosh P-15, engl.

As far as four-axle ARFFs go, it all started with the P-15. On behalf of the government, Oshkosh developed this vehicle, designated A/S 32P-15. Based on this basic concept, numerous civilian variants with the designations M-12, M-15 and M-23 followed later.
These designations meant:

M-12: 3000 gallons, 12000 liters of water
M-15: 4000 gallons, 15000 liters of water
M-23: 6000 gallons, 23000 liters of water

At the time of development in 1977, seven commercial versions had already been built and delivered to customers. At the same time, other units were in the assembly or testing process.

Oshkosh P-15