BE20 pilot flying reported the co-pilot side windshield shattered inflight and to get the controller's attention on a congested frequency; requested priority handling. As the windshield still seemed structurally sound and there was no loss of pressurization; the pilot opted to perform an air turnback.

Narrative

Just a few minutes prior to our inflight co-pilot windshield shattering at 29;000 ft.; a Part 121 aircraft had [requested priority handling]; requesting expedited handling into ZZZ for a passenger medical event. After typical customary quiet period on the frequency after [requesting priority handling]; the controller had come back and was issuing rapid fire instructions to aircraft that had lagged during the ZZZ Part 121 [priority] handling.Per the King Air abnormal checklist for a shattered windshield; I needed to descend immediately to at least 25;000 ft. and achieve a differential pressure on airframe of between 2.0 and 4.6 psi. At FL290 we had 6.8 psi on airframe which is max differential. I waited 30 seconds and could not break into the frequency due to congestion by controller. After reviewing traffic screen; and seeing no traffic at within 12 miles; I squawked 7700; hit IDENT and commenced a descent at over 2;000 FPM. I hit the IDENT two more times and after about 900 ft. of descent - another 30 seconds from commencement of descent; almost 2 minutes after windshield event - I finally was able to [request priority handling] and advise we were in descent for FL220. Controller initially thought we were the Part 121 [priority handling]; which I immediately clarified. He finally acknowledged our descent and gave us FL220. I told controller I squawked 7700 only to get his attention of descent; and that I did not require any further assistance. He told me to squawk the prior code; which we immediately did.We were handed off to another controller and requested direct ZZZ1 - 180-degree turn - and intended to return to origin airport of our flight. We were given 17;000 ft. and ultimately went down to 12;000 ft. At this point; we had the cabin differential pressure below 1.0. At no point did we have any loss of pressure in aircraft. The controller cleared us direct to the ZZZ2 Airport; I clarified we were returning to our origin airport. She clarified that we had [requested priority handling]; and I indicated we [requested] for need to descend and the congestion of the frequency having difficulty communicating with controller. I pointed out prior controller gave us our old squawk and we were returning to point of origin. The integrity of windshield was sound; as designed by the two-ply construction; and without any loss of pressurization I was comfortable returning to origin at an unpressurized altitude.