After a long career in aerospace engineering, including much time working with subcontractors, I would say letting separate contracts for designing and for building is a prescription for disaster.

We would always let one contract for a deliverable product, from design to delivery at the loading dock being the subcontractor's responsibility. The contract would specify requirements in great detail, including dimensions, weight, loads, power, and performance. It would also specify the testing to be performed to verify that all of these requirements were met, and we reserved the right to pre-approve the testing plan and to witness the tests to assure everything was verified to the letter of the contract.

It was okay for the subcontractor to sub-subcontract parts of his deliverable product, but he was responsible for every aspect of the entire package. So there was no opportunity for finger-pointing between contractors.