The planforms of its midmounted wings were nearly indistinguishable, with straight center sections extending to just outboard of the inner engines and identically tapered outer panels ending in rounded tips. The Lancaster’s wing area was slightly larger, but its wing loading was higher, its gross weight of 68,000 pounds exceeding that of the Halifax by almost 14,000 pounds. Initially, both airplanes had Rolls-Royce Merlins, an engine most famous, at least to Americans, for its use in the P-51. In the Mustang, the Merlin was rated at 1,650 hp; in the Halifax, with a different supercharger, it put out only 1,300 hp and was eventually replaced by Bristol Hercules sleeve-valve radials of 1,650 hp. Concern about a possible shortage of Merlins caused the Hercules to be fitted to the Lancaster as well, but the arrival of plentiful Packard-built Merlins from the United States caused only 300 of that variant to be built.