Carefully-conducted tests had proved that an Imperial Airways' Empire flying-boat could achieve a transatlantic crossing only if its entire payload consisted of fuel. Since it is well known that an aircraft can be flown at a much greater weight than that at which it can take off from the ground, Robert Mayo proposed that a small heavily loaded mailplane be carried to operational altitude above a larger 'mother plane' and then released to complete its long-range task. The proposal was accepted by the Air Ministry and Imperial Airways, who jointly contracted Shorts to design and build such a composite unit. The Short S.21 Maia, the lower component, was a slightly enlarged and modified version of the Empire 'boat; the Short S.20 Mercury, the upper long-range unit, was a new high-wing twin-float seaplane with four 340 hp (254 kW) Napier Rapier H engines giving a cruising range of 3,800 miles (6116 km) with 1,000 lb (454 kg) of mail.
The first airborne separation took place on 6th February 1938, and after a number of experimental flights Mercury was air-launched over Foynes on 21st July to fly nonstop the 2,930
miles (4715 km) to Montreal in 20 hours 20 minutes with a 600 lb (272 kg) payload. On 6th October 1938 Mercury was launched over Dundee to establish an as yet unbroken non stop international seaplane distance record of 5,997.5 miles (9652 km) to the Orange River, South Africa. However, the outbreak of war ended experimentation, and Mercury was eventually broken up at Rochester and Maia destroyed by enemy action during May 1941.