It's part of the undercarriage. It's a bit dark but you can nearly see the same thing under the starboard wing.
It goes back to Martlesham Heath complaining that the take-off run was too long and requesting an increase in wing incidence.
That would have caused a nose-down cruise like the Whitley, and tooling was already well advanced for production, so they fitted what was basically a big crate between the top of the undercarriage and the wing mountings.
It worked but was notoriously fragile.