The first prototype was converted from a Hamilcar Mark I. Two Bristol Mercury radial piston engines, capable of producing each were added to the wings of the glider, and the wings and fuselage were strengthened so they could take the weight of the engines. Extra controls were added to the cockpit and duplicated in the two pilot positions, although space restrictions meant that the glider could only be started from the rear seat, and fuel tanks were added to the wings, with the possibility of a third being carried in the fuselage. These additions increased the glider's weight to , but its other dimensions remained unchanged, including the carrying capacity in the fuselage. The first flight of the Mark X, under its own power, took place in February 1945 and the initial flight and further trials showed that the glider operated as had been expected. With engines installed, a Mark X could be towed by a fully loaded Halifax and achieve an operational radius of approximately . However, if the glider itself was fully loaded and took off on its own power, it could not maintain height even at full power; this resulted in a decision to decrease the amount of cargo it could carry under its own power, which in turn decreased its weight to . Two Hamilcar Mark Is were converted for initial trials, and when these proved satisfactory a further eight Mark Is were converted and ten Mark Xs built from scratch; any further orders were cancelled when the conflict ended in August 1945, although further tests were conducted in the United States. One other Hamilcar variant was proposed, although it never went into production or appeared to go further than the design stage. This was a proposal to mate a Lockheed P-38 Lightning fighter to the top of a Hamilcar, as the fighter would provide enough power to keep both aircraft in flight and relieve the glider pilots of the task of controlling the glider until it cast off to land.