 Before working for the LB&SCR, Billinton worked for the Midland Railway's locomotive department, hence he had got used to 0-4-4T designs which that company used for similar work which the D3's were built for.The design centred on the idea of being an improved version of William Stroudley's D1 tanks, the D3 intending to be replace the D1s on certain duties.The design shared spare parts with Billinton's other designs, the cylinder's design being shared with the C2 goods engines, the boilers with the E4 radial tanks.