Except for the YB-49, I worked at Northrop during all these aircraft efforts and, while I was not working these because of my efforts on other aircraft programs such as the F-18L and black programs, I can comment about them. Two other programs should be included in the list, one is the already mentioned F-18L, and the other is the Navy Advanced Tactical Aircraft, ATA (A-12). Lets start with the YF-17.
YF-17. On its own money, Northrop developed a follow-on fighter to the F-5 however the US government was anticipating forcing foreign sales to its Light Weight Fighter Program (LWF) and squelched sales of the Northrop effort. Northrop decided to compete the LWF but was already at a significant disadvantage due to the requirement to use the same engine as the F-15 and have a single engine. While the YF-17 performed well (it was aerodynamically much better and was lighter than the fatter follow-on F-18a/b), it had no chance and the F-16 was given the contract. In my opinion, this was a major error. The PW engine was immature and substantial losses of the F-16 due to engine failure (been unable to fine data on early engines), and follow-on dedicated F-16 engine designed, eradicated any benefit of the concept of similar engines to the F-15. In addition, the F-16 had very limited growth and its inability to carry the AIM-7 meant that it was only a day VFR fighter for thirteen years until the introduction of the AMRAAM, a considerable disadvantage in cloudy Europe.
F-18L. In the LWF program, the Navy demanded a lead for the Navy version be an airframe manufacturer with Navy experience (something not required with the F-35). As such, Northrop teamed with McDonald Douglas. The deal was that Northrop would be the lead for all much lighter Land based F-18, denoted by the F-18L model, and MD would be the lead for Navy versions. When the LWF contract split, an issue arose. Against the initial contract agreement, MD competed with the F-18L. A law suit was ensued and MD ended up giving some money to Northrop but taking over the entire program, certainly an uncomfortable conclusion for Northrop but basically forced by business decisions. Definitely screwed.
F-20. Another self-financed attempt by Northrop to replace the very successful F-5 was the F-20. An excellent aircraft, was relatively cheap, very efficient, and had great maneuverability. However, the F-16, the AF and government again swashed already agreed-to sales. Northrop was unable to legally challenge the government due to the risking B-2 contract. Another screw job.
Navy ATA. The US Navy requested proposals for a stealthy attack aircraft. Northrop and McAir-General Dynamics were competing. Northrop's proposal was a mini-B-2 with two engines. McAir came back at a billion dollar less bid. Northrop, with B-2 experience, already knew what the cost of building a stealth aircraft was going to be and would not meet McAir cost and withdrew and McAir got the contract. Three years later the program was cancelled due to massive cost overruns. Was Northrop screwed? Well let's just say the Government use poor decision making techniques.
YF-23. This competition was tough to lose. The YF-23 met all maneuverability requirements without the complex thrust vectoring, was faster in super cruise, and had better stealth characteristics. Plus, it looks totally cool. The proposal evaluation team selected the YF-23 as the technical preference. However, Lockheed Martin was considered a better at program management than Northrop (Northrop was struggling with the highly complex B-2 at the time) and was given the contract. As it turns out Lockheed Martin was no better, possible worst, in developing the F-22 and F-35. I must admit I don't know how much the government adds to the difficulty of these program but I suspect it is not little. In general, a poor decision, I think.
F-35. Northrop competed with Boeing and Lockheed for the JSF program. The Northrop proposal used a separate lift engine for the V/STOL version a la Russian Forger. In my opinion, an idiotic idea, and I would not have selected this. Boeing proposal was too ugly to win. So Lockheed Martin won. The jury is still out.
Northrop has had some great planes but some bad breaks, some political.
Northrop has had some great successes. The F-5 program was a great success with over 3000 T-38/F-5 units built. Of course the B-2, 50% (?) of the F-18, Global Hawk, and many others with the procurement of other companies such as Grumman and TRW, have made it quite profitable. Its expertise in drones places it in a good position for future growth.