Dhaka, March 14 -- A converted passenger jet is shaping up as an unexpected competitor to Boeing's next-generation cargo aircraft, the 777-8F, and it is built from Boeing's own airframes.

Leasing giant AerCap and Israel Aerospace Industries are converting retired Boeing 777-300ER jets into large freighters dubbed the "Big Twin," offering cargo operators a cost-effective alternative to the upcoming Boeing 777-8F.

The converted aircraft can carry 33 main-deck pallets and roughly 819 cubic meters of cargo volume, slightly exceeding the 777-8F's capacity, and is optimized for bulky, lighter shipments such as e-commerce packages, where volume often matters more than payload weight.

Cost presents a compelling case. A Big Twin conversion is e...