 de Havilland was approached to produce an airframe for the H.1 as insurance against Germany using jet bombers against Britain; this was considered more important than de Havilland's own suggestion of a high-speed jet bomber.Its first design, designated as the "DH.99", was an all-metal, twin-boom, tricycle undercarriage aircraft armed with four cannon.The use of a twin boom enabled the jet pipe to be kept relatively short, which avoided the power loss that would have occurred if a long pipe was used, as would have been necessary in a conventional fuselage.