The Hindenburg could home on navigational beacons and travel low enough when over land to get pretty good navigational fixes. At sea they could exchange info with any ships sited. My uncle taught navigation for the USAAF for a while early in the war then flew his bomber over to Great Britain via the southern route. Going over to Africa he suspected an unforcassed head wind and had the pilot take them up high enough to get a star shot. Since you can't do this accurately through glass it's not exactly a comfortable operation. Of course not finding Africa (as one of planes in the stream with him didn't) wouldn't be comfortable either. The flying from Africa to Wales they spotted a lighthouse off the starboard side. There weren't suppose to be any so a bit of checking indicated it was in Portugal and they had encountered an unexpected cross wind. The navigational beacon they were suppose to be following got turned off for some reason. They did eventually make contact with a field in Wales (not the one they were suppose to land at which was just as well as it was fogged in) and managed to land safely. Note that planes bombed Switzerland and the Republic of Ireland accidentally as well. The AA guns were concentrated around the coast and coastal defenses from what I've read. Some may have been focused around some installations near the coast as well I'm not sure about factories but suspect not much. But then finding the factories would have been a bit of a chore as well. Also if you bomb from 5-6k feet it means dropping down and then going back up to a cruising altitude which burns more fuel. Again at what point in the war this would occur is critical. Very early there wouldn't be much opposition in terms of either fighters or AAA. Midterm it would be pretty difficult later it's harder to say. Many of the fixed defenses were gone or unmanned but the intell network was pretty good and the USN had a lot of ships with radar and a fair number of carriers in the Atlantic. Put a line of radar pickets backed up by 2 or 3 CVE's and the raid could be in serious trouble before it even reached the American coast.