The DO-178B/C explicitly does not specify how verification is done but describes the properties that are expected of the verification evidences that you submit to the certification authorities; so formal verification is perfectly fine as one item on your verification check list. In particular DO-333 amends DO-178C with specific topics concerning formal methods.

For example Astree [1] has been developed for decades now, with Airbus as one of the major sponsors.

[1] http://www.astree.ens.fr/