The Monospar ST-25 was a low-wing cantilever monoplane with a fabric-covered metal structure. The monospar name came from the use of a single spar in the wing structure, that had been developed by H J Stieger. The cabin was enclosed with five seats. It was based on the GAL Monospar ST-10, with the addition of folding seat for a fifth passenger, extra side windows, and the addition of a radio receiver. On 19 June 1935, the prototype (G-ADIV) made its first flight at Hanworth Air Park. It was designated Monospar ST-25 Jubilee, to honour the 25th anniversary of the reign of King George V.
One Monospar ST-25 Jubilee with large single fin and two Niagara II engines, later converted to the prototype Monospar ST-25 Universal, with twin fins.
Monospar ST-25 Ambulance
Variants of both Monospar ST-25 Jubilee and ST-25 Universal, with a large door on starboard side to allow a stretcher to be loaded.
Monospar ST-25 Universal
(1936-1939) Twin fin and twin rudder. 29 built, including the conversion of the De Luxe.
Monospar ST-25 Freighter
A variant of the Monospar ST-25 Universal, with a large freight door but without the passenger seating.
One modified Monospar ST-25 Jubilee, fitted with two Cirrus Minor I engines in 1936.
One experimental aircraft based on the Monospar ST-25 Universal. A new fuselage was built containing a pressurized section with two seats. Its purpose was to test possible pressurization systems for a proposed airliner, the GAL.40. The GAL.41 flew for the first time 11 May 1939, and was grounded in 1941.