During the Cold War the NATO and the "Union of peace and socialism" (Commonly known as Warsaw Pact) were in an arms race - with the (perceived) Missile Gap the most known example, though there was also concerns about the raw numbers of tanks or infantry.
During that time the US called for increases (Option 1 & 2).
During that time the term "NATO Burden-Sharing" was established, which quickly lead to the perception the European Nations do not pay their fair share for the level of protection they receive (Whether that is true is up for discussion)
When the Cold War was over the NATO shifted it's concerns and started to worry that the budget constraints the EU "enforces" could lead to decreased European spending: Implications of European Integration for Allies' Defense Spending
Since the debt crisis the European countries have taken up austerity measures which again concerns the US as seen in The European Debt Crisis and American Security Policy.
During Operation Unified Protector some European countries that pledged to support somehow did not participate in the air strikes which lead Defense Secretary Gates to call for more spending.
In the past, I’ve worried openly about NATO turning into a two-tiered alliance: Between members who specialize in “soft’ humanitarian, development, peacekeeping, and talking tasks, and those conducting the “hard” combat missions. Between those willing and able to pay the price and bear the burdens of alliance commitments, and those who enjoy the benefits of NATO membership – be they security guarantees or headquarters billets – but don’t want to share the risks and the costs. This is no longer a hypothetical worry. We are there today. And it is unacceptable.
I am the latest in a string of U.S. defense secretaries who have urged allies privately and publicly, often with exasperation, to meet agreed-upon NATO benchmarks for defense spending. However, fiscal, political and demographic realities make this unlikely to happen anytime soon, as even military stalwarts like the U.K have been forced to ratchet back with major cuts to force structure. Today, just five of 28 allies – the U.S., U.K., France, Greece, along with Albania – exceed the agreed 2% of GDP spending on defense.
Especially since the Ukraine crisis almost everyone seems to call for more spending.
Transatlantic Forum: Rebalancing and Reinforcing the Transatlantic Bond
The United States will need to balance between encouraging European allies to assume greater responsibility for their region’s security and maintaining the level of U.S.
investment and engagement required to credibly deter threats to Europe and assure partners and allies on NATO’s southern and eastern borders.
A few months later Nobel Peace Prize Winner President Obama also urged for more European Participation
But one of the things that I’ve also said in the past and will repeat again -- and I think Secretary General Rasmussen agrees with me here -- is that if we’ve got collective defense, it means that everybody has got to chip in. And I have had some concerns about a diminished level of defense spending among some of our partners in NATO -- not all, but many. The trend lines have been going down. That’s understandable when you have an economic crisis and financial crisis, and many countries are going through fiscal consolidation. But the situation in Ukraine reminds us that our freedom isn’t free, and we’ve got to be willing to pay for the assets, the personnel, the training that’s required to make sure that we have a credible NATO force and an effective deterrent force.
So one of the things that I think, medium and long term, we’ll have to examine is whether everybody is chipping in. And this can’t just be a U.S. exercise or a British exercise or one country’s efforts. Everybody is going to have to make sure that they are engaged and involved. And I think that will help build more confidence among some of those border states.
During all those papers and speeches the UK was singled out two or three times as doing more than other European nations (Though the eastern countries like Latvia spend more).
However I was unable to find any recent proposals of the US helping any country to meet it's spending targets - on the contrary they urge them to take it up for themselves.