Aero Spacelines, then based at Van Nuys, California, were quick to appreciate the potential of outsize cargo aircraft that would be able to airlift large booster rocket stages being employed in the American space programme, as well as for the transport of aircraft assemblies, oil drilling equipment, and other items too large to be carried by any then existing aircraft. Work started in 1961 on the conversion of a Boeing B-377 Stratocruiser to fulfil such a role: the fuselage was extended by 5.08m aft of the wing, and a new 'bubble' structure was added over the top of the fuselage to allow the loading of items up to 6.02m in diameter. The resulting B-377PG Pregnant Guppy was flown for the first time on 19 September 1962, and was used from the summer of 1963, under contract to NASA, for the transport of space programme hardware.