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.

It was based on the GAL Monospar ST-10, with the addition of a 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.[2] Data from Jackson, 1973General characteristics Performance