The Heinkel HD 21 was a trainer built in Germany during the 1920s. It was a conventional single-bay biplane with staggered wings braced with N-type interplane struts. The main units of the fixed, tailskid undercarriage were linked by a cross-axle, and the aircraft had three open cockpits in tandem, although one of these was typically faired over. The HD 29 of 1925 was essentially similar, but was purely a two-seater.