A Redditor noticed how close the two were when looking at all-time stats on Box Office Mojo, and the fact that the last Avengers movie made more than $600 million domestically means it stands to reason Age of Ultron will give Lee the $400 million or so he needs to catch up to Spielberg.
Of course, the "victory" may be short lived, since Spielberg will be producing Jurassic World later this year. While it's considered a given that Age of Ultron will outgross that film, a respectable showing by the Jurassic Park sequel could bridge the gap between what Avengers makes and what Lee's lead would be as a result.
What's arguably the most interesting about this is that Lee has been credited as a producer on only 32 films to Spielberg's 65, with Lee's average domestic gross being almost double Spielberg's. Lee is also one of only three producers whose films have grossed $5 billion or more domestically -- the third being Lucasfilm's Kathleen Kennedy with 57 films to her credit.
Of course, Lee isn't directly involved with most of those films, and is given a producer's credit primarily for his work on the comics. Marvel Studio chief Kevin Feige, however, who seemingly has the ultimate say in the Marvel Cinematic Universe, is nipping at the heels of $5 billion with only 23 movies on his resume.
For context, Feige is the fourth-highest-grossing producer on the list and Arnon Milchan (the #5 candidate, with about $200 million less than Feige has to his name) has produced 115 movies -- five times the number that Feige has.
It may be another 2-3 years before this particular record settles into any kind of stability and produces a meaningful indicator of who is the most dominant filmmaker of all time. Nearly everyone in the top ten has a film expected to make $500 million or so in the next year, and the #1 spot (Spielberg with $6.8 billion) is separated from the #4 spot (Feige with $4.9 billion) by little more than what today's blockbuster filmmakers would consider "a good year."