From http://www.aviastar.org/air/england/handley_heyford.php

First flown in prototype form in mid-1930, the Heyford was the last of
the RAF's long-range biplane night bombers. It was powered in Mk I form
by two 391.2kW Rolls-Royce Kestrel III engines and in the Mk II and Mk
III by 428.5kW Kestrel VI. The Heyford was an equal-span biplane with
staggered wings: the upper wing centre-section rested on top of the
fuselage, while the lower was positioned well below the fuselage,
connected to the under-fuselage by N-type struts. The inner interplane
struts supported the engine mountings. An interesting feature of the
design was that bombs of various sizes were carried inside the thickened
centre-section of the lower wing, each bomb being carried in a separate
cell closed by spring doors. The fixed landing gear comprised two large
wheels faired into the lower wing.

A total of 124 Heyfords were built, made up of 38 Mk I and IA, 1
intermediate Mk IA/II, 16 Mk II and 71 Mk Ill - these figures being
adjusted to take into account changes made from the original production
orders. Heyfords served with heavy-bomber squadrons from 1933 to 1939,
giving way to more modern monoplanes of World War II-type.