The -415 is based on the original CL-215, the first purpose-built water bomber. When Canadair designed it in the 60s, radial engines were both plentiful and relatively inexpensive, so they made it so the CL-215 used the entire R-2800 power pod (engine, nacelle, propeller, etc) from then-surplus A-26 Invaders. This meant that operators could obtain power pods from surplus dealers and have spares on hand.

Over time though, that caught up with the original design and as radials became more and more rare operators started requesting alternative power.

The first step Canadair took was to swap out the R2800s for then-new Pratt and Whitney Canada PW120 turboprops. This and a few other mods created the CL-215T, which is the designation for all converted aircraft. The swap mainly consisted of the engines and nacelles, the fuel system, and a few aerodynamic tweaks like the finlets on the horizontal tail.

Canadair then took to redesigning the water bomber to better use the new powerplant. With the -415 they subsantially changed the internal design, including such amenities as powered controls (until then they were pure cable operated), air conditioning, and a revised water dump system that allowed for sequential drops instead of one large water drop.

The new -515 adds a full EFIS cockpit, increased automation, and new sensors.