Production of the York proved difficult to speed up, due to shortages of key materials. Moreover, Avro was also obligated to place a high priority on the manufacturing and refinement of the Lancaster. Officials had also judged that there was no requirement for large numbers of Yorks at that time. By the end of 1943, only the four prototypes and three production aircraft had been manufactured, but production was scheduled to rise to three aircraft per month throughout 1944. Early production Yorks were principally used as a VIP transport aircraft; notably, the third prototype, "LV633" being luxuriously fitted out and becoming the personal transport of Winston Churchill.